Journal of Sol-Gel Science and Technology 28, 255–265, 2003 c 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.
Synthesis, Characterization and Applications of Immobilized Iminodiacetic Acid-Modified Silica ISSA M. EL-NAHHAL,∗ FARID R. ZAGGOUT AND MONA A. NASSAR Department of Chemistry, Al-Azhar University, P.O. Box 1277, Gaza, Palestine
[email protected]
NIZAM M. EL-ASHGAR Department of Chemistry, The Islamic University of Gaza, P.O. Box 108, Gaza, Palestine JOCELYNE MAQUET AND FLORENCE BABONNEAU Laboratoire de Chimie de la Mati`ere Condens´ee, Universit´e Paris VI, 4 place Jussieu, 75252 Paris, France MOHAMED M. CHEHIMI ITODYS, Universit´e Paris VII—Denis Diderot, associ´e au CNRS (UMR 7086), 1 rue Guy de la Brosse, 75005 Paris Received January 24, 2003; Accepted June 17, 2003
Abstract. Porous immobilized iminodiacetic acid modified silica of the general formula S (CH2 )3 N(CH2 COOH)2 , (where S represents [Si O]n siloxane network) has been prepared by replacement of the iodide in 3-iodopropyl modified silica with diethyliminodiacetate. The immobilized-diethyliminodiacetate ligand system (S-DIDA) was then hydrolyzed by hydrochloric acid to produce the immobilized iminodiacetic acid ligand system (S-IDA). The iodo functionalized modified silica (S-I) was prepared by polycondensation of Si(OEt)4 and (MeO)3 Si(CH2 )3 I. The XPS and CP/MAS 13 C NMR spectra showed that not all iodine atoms are replaced and that the hydrolysis of ethyl acetate groups are incomplete upon treatment with HCl. The immobilized iminodiacetic acid ligand system exhibits high potential for the uptake of various di- and trivalent metal ions such as (Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ). Complexation of the iminodiacetate ligand system for the metal ions at the optimum conditions was found in the order: Cu2+ > Fe3+ > Ni2+ > Co2+ > Mn2+ > Zn2+ . Stability studies of the iminodiacetate ligand system showed that a degradation of the siloxane network and leaching of some species occurred upon treatment with strong acid and base aqueous solutions. Keywords: metal uptake, iminodiacetic acid, diethyliminodiacetate ligand, modified silica, immobilized silica
1.
Introduction
Many functionalized silica based materials have been made either by anchoring the organofunctional groups on silica [1–3] or incorporation into the sol-gel materials [4–10]. These functionalized materials include, ∗ To
whom all correspondence should be addressed.
amines [4, 5], thiol [4], phosphines [8], glycinate [9], and iminodiacetate [10] and others [11, 12]. Such materials have superior properties over organic resins, due to their high thermal, hydrolytic, mechanical stability in addition to lack of swelling in solvents [13, 14]. These immobilized ligand systems exhibit a great potential for extraction, recovery and separation of metal ions from aqueous solution [4–10], stationary
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phases in chromatography [9] and as supported ligands for catalysts [8, 15]. Characterization of these polymeric systems has been recently carried out using highresolution solid state NMR techniques [16–20], as well as other spectroscopic methods [21–24]. Particularly, X-ray photoelectron spectroscopy (XPS) has proved effective in characterizing the change in chemical structure upon acid treatment of polysiloxane-immobilized monoamine ligand [24]. Moreover, it is now an established analytical technique in the field of sol-gel science and technology [25–27]. Despite the fact that the immobilized iminodiacetate ligand system is reported recently [10]; its structure is still not well identified. In this work the immobilized iminodiacetate ligand system was prepared, and characterized using variety of physical chemistry techniques including, high resolution solid state 13 C CPMAS NMR, X-ray photoelectron spectroscopy (XPS), thermal analysis (TGA and DTG) as well as infrared spectroscopy (FTIR). The uptake of metal ions (Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ and Mn2+ ) of the immobilized iminodiacetate ligand system was also examined and investigated at different conditions and parameters e.g. shaking time, pH, and particle size. 2. 2.1.
Experimental
(I A /s A ) %A = × 100 (In /sn )
Reagents and Materials
Tetraethylorthosilicate, 3-chloropropyltrimethoxysilane, and iminodiacetic acid, were purchased from (MERCK) and used as received. Acetone, diethyl ether and methanol (spectroscopic grade) were used as received. Metal(II) solutions of the appropriate concentration were prepared by dissolving the metal(II) chloride (analar grade) in deionized water. Different pH ranges were controlled using buffer solutions. pH (1–2) was obtained using HCl/KCl buffer, pH (3.5–6) by using acetic acid/sodium acetate and pH > 7 by using ammonia/ammonium chloride. 2.2.
were performed under the same Hartmann-Hahn match with RF fields around 42 kHz and contact times of 1to-2 ms. Proton decoupling was always applied during acquisition with a repetition time of 5 s. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Surface Science Instrument (SSI) spectrometer, SSX 100 model, equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The X-ray spot size was 1000 µm. The pass energy was set at 150 and 100 eV for the survey and the narrow scans, respectively. The step size was 1.12 eV for the survey spectrum and 0.096 eV for the highresolution spectra. Charge compensation was achieved with a flood gun of 1.5 eV electrons using standard procedures for this spectrometer. The pressure in the analysis chamber was ca. 5 × 10−9 mbar. Data processing was achieved with a Winspec software, kindly supplied by the Laboratoire Interdisciplinaire de Spectroscopie d’Electrons (LISE, Namur, Belgium). Spectral calibration was determined by setting the C1s ester component or the C1s COOH component at 289 and 289.2 eV, respectively. The surface composition was determined using the manufacturer’s sensitivity factors. The fractional concentration of a particular element A (%A) was computed using:
General Techniques
Analysis for carbon, hydrogen, and nitrogen were carried out, using an Elemental Analyzer EA 1110-CHNS CE Instrument. 13 C CP-MAS Solid State NMR experiments were recorded on MSL 300 or MSL 400 Bruker Spectrometers at 75.5 and 100.7 MHz respectively. The samples were spun at 4–5 kHz using 7-mm rotors. Experiments
where In and sn are the integrated peak areas and the sensitivity factors, respectively. Thermogravimetric analysis was carried out using a Perkin-Elmer Thermogravimetric Analyzer TGA6. The concentrations of metal ions in their aqueous solutions were measured using a Perkin-Elmer AAnalyst100, spectrometer. The infrared spectra for the materials were recorded on a Perkin-Elmer FTIR, spectrometer using KBr disk in the range 4000 to 400 cm−1 . All pH measurements were obtained using HM-40 V pH Meter. All ligand samples were shaken in aqueous metal ion solutions using an ELEIA-Multi Shaker. 2.3.
Methods of Preparation
2.3.1. Preparation of 3-Iodopropyl Modified Silica (S-I). The 3-iodopropyltrimethoxysilane was firstly prepared [11, 12] by adding sodium iodide to stirred solution of the 3-chloropropyltrimethoxysilane.
Immobilized Iminodiacetic Acid-Modified Silica
Table 1. Elemental analysis data for the 3-iodopropyl modified silica (S-I). %H
%Cl
%I
C/X∗
Modified silica
Element
%C
S-I
Expected
10.5
1.8
0.0
37.2
3.0
Found
9.0
2.2
0.0
32.3
3.0
X∗ = I + Cl.
Iodopropyl modified silica was then prepared as previously reported [11] by adding 3-iodopropyltrimethoxysilane to stirred solution of tetraethylorthosilicate in the presence of HCl as a catalyst. The elemental analysis for the 3-iodopropyl modified silica is given in Table 1. 2.3.2. Preparation of Iminodiacetic Acid Modified Silica (S-IDA) [10]. The immobilized iminodiacetic acid modified silica (S-IDA) was prepared using three steps reaction as follow: 2.3.2.1. Preparation of Diethyliminodiacetate (DIDA). The diethyliminodiacetate (DIDA) was prepared as described previously [10] by treatment the iminodiacetic acid with ethanol in the presence of thionyl chloride. 2.3.2.2. Immobilization Step. Diethyliminodiacetate modified silica was prepared by adding diethyliminodiacetate (9.45 g 0.05 mol) to 10 g of iodopropyl modified silica in 50 cm3 of toluene. The mixture was stirred and refluxed at 95◦ C. The product was filtered off, washed with 0.025 M NaOH, water, methanol and diethyl ether, then dried in vacuum oven (0.1 torr) at 90◦ C for 12 h. The elemental analysis for the diethyliminodiacetate modified silica is given in Table 2. 2.3.2.3. Hydrolysis Step. Immobilized diethyliminodiacetate modified silica (5.0 g) was hydrolyzed by refluxing the ligand system with 150 cm3 of 2.0 M HCl for 6 h with stirring. The solid material was then filtered,
washed with 0.025 M NaOH aqueous solution, water, methanol and diethyl ether. The material was dried in vacuum oven (0.1 torr) at 90◦ C for 12 h. The elemental analysis for the iminodiacetic acid modified silica is given in Table 2. 2.3.3. Metal Uptake Experiments. A 100 mg of functionalized iminodiacetic acid ligand system was shaken with 25 cm3 , 0.02 M of aqueous solution of the appropriate metal ions (Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ) using 100-cm3 polyethylene bottles. Determination of the metal ion concentration was carried out by allowing the insoluble complex to settle down and appropriate volume of the supernatant was withdrawn using a micropipette then diluted to the linear range of the calibration curve for each metal. The metal ion uptake was calculated as mmole of Mn+ /g ligand. Each study was performed at least in a triplicate. Metal uptake was examined under various factors including the pH, shaking time, particle size and shaking effect. 3. 3.1.
Results and Discussion Preparation of 3-Iodopropyl Modified Silica (S-I)
The 3-iodopropyl modified silica was prepared as previously reported [11], by hydrolytic condensation of 3-iodopropyltrimethoxysilane and tetraethylorthosilicate (TEOS) in the ratio of 1:2 (Scheme 1). The elemental analysis results of the products are given in Table 1. The lower carbon and iodine percentages for the 3-iodopropyl modified silica S-I than the expected values, indicating the formation of small soluble oligomers, which are lost during the washing, process [28]. This is due to the different rate of hydrolysis of the two alkoxy silane agents [(C2 H5 O)4 and (CH3 O)3 Si(CH2 )3 I)]. The methoxy groups are hydrolyzed faster than the ethoxy groups. This will lead to self condensation of 3-iodopropyltrimethoxysilane
Table 2. Elemental analysis data for the immobilized diethyliminodiacetate ligand system (S-DIDA) and its hydrolyzed form (S-IDA). Modified silica S-DIDA S-IDA
Element
%C
%H
%N
C/N
Expected
22.6
3.4
2.4
11.0
Found
18.8
4.1
1.73
12.7
Expected
18.10
3.75
2.85
Found
16.7
3.5
1.40
257
O (CH3O)3Si(CH2)3I
+ 2 Si(OC2H5)4
H2O/MeOH
HCl
O O
7.40
S-I
13.9 Scheme 1
Si(CH2)3I
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El-Nahhal et al.
O O
CH2
C OC2H5
CH2
O
O
N(Et)3
Si(CH2)3I + HN
O
O
O
C OC2H5
Si
CH2
C OC2H5
S-DIDA O
O O
C OC2H5
N
O
O DIDA
S-I
CH2
O
HCl
CH2 C OH Si-CH2-CH2-CH2 N CH2 C OH
O S-IDA
O
Scheme 2
which result in the formation of low molecular weight products, which were washed away. 3.2.
Preparation of the S-DIDA and S-IDA Ligand Systems
The immobilized iminodiacetate ligand system was prepared by chemical modification of the preformed 3-iodopropyl modified silica (S-I) with the diethyliminodiacetate (DIDA) in the in presence of triethyl amine (Scheme 2). The immobilized diethyliminodiacetate was then hydrolyzed by hydrochloric acid to produce the carboxylic acid form (S-IDA) (Scheme 2). The elemental analyses are given in Table 2. The lower C and N percentages of S-DIDA than those expected is probably due to incomplete substitution reaction of the iodide. This is probably due to steric hindrance of the large diethyliminodiacetate molecules. Evidences were confirmed by the solid state 13 C NMR and XPS results discussed latter. The elemental analysis of S-IDA shows that there is a decrease of carbon percentage due to the hydrolysis of ethoxy groups into the acid form. This was confirmed by the XPS results discussed later.
Figure 1. (a) 13 C CP-MAS NMR of S-I. (b) of S-DIDA. (c) 13 C CP-MAS NMR of S-IDA.
CP-MAS 13 C NMR Spectra
The CP-MAS 13 C NMR spectra for the immobilized S-I, S-DIDA and S-IDA ligand systems are shown in Fig. 1. The assignments of the 13 C NMR spectra for these immobilized ligands were based on spectral data
CP-MAS NMR
of similar systems reported in the literature [16, 17]. O O
3.3.
13 C
2 3 1 Si-C H2-CH2-CH2-I
O
The CP-MAS 13 C NMR spectrum of the 3Iodopropyl modified silica (S-I) (Fig. 1(a)) displays a shoulder and two peaks maxima position at ≈10.5(sh),
Immobilized Iminodiacetic Acid-Modified Silica
O 1 2 3 Si -CH2-CH2-CH2 N
O
14000
(a) O
12000 10000
Intensity (cps)
14.5 and 27.8 ppm for the three methylene carbons C1, C2 and C3. The signals at 18.7, 61.3 and 48.6 ppm were identified for the unhydrolyzed OC2 H5 and OCH3 groups respectively. The 13 C CP-MAS NMR spectrum of the immobilized diethyliminodiacetate ligand system (S-DIDA) is given in Fig. 1(b). The spectrum shows three signals at 9.9, 27.3 and 48.2 ppm due to the three methylene carbons C1, C2, and C3. The doublet signal at 172 ppm is due to the carbonyl C5. The presence of doublet for the carbonyl indicates that there are two resonances, free carbonyl and hydrogen bonded carbonyl. The carbon signals at 18.7 and 58(sh) ppm are assigned to C7 and C6 respectively. The signal at 60 ppm is due to C4.
8000 6000
I
C 4000
Si
N
2000 0 0
200
400
600
800
1000
1200
1000
1200
Binding energy (eV) 14000
O 5 6 7 4 CH2 C OCH2CH3
(b) O
12000
10000
The presence of strong signal at 14.9 ppm, which assigned for C2 of the 3-iodopropyl modified silica, indicates the presence of unreacted iodopropyl groups. The CP-MAS 13 C NMR spectrum for the immobilized iminodiacetatic acid ligand system (S-IDA) is given in Fig. 1(c). The spectrum shows three carbon signals at 10.6, 26.8 and 48.4 ppm due to the three methylene carbons, C1, C2 and C3 respectively. The presence of two signals at 18.0 and 58.5 ppm corresponds to CH3 and OCH2 provide evidence for the presence of some unhydrolyzed ( OCH2 CH3 ) groups even after treating the immobilized S-DIDA with HCl. The signals at 60.0 and 169.0 ppm are assigned due to C4 and C5 respectively.
O O
1 2 3 Si-CH2-CH2-CH2 N
O
O 5 4 C OH CH2
8000
6000
C
4000
Si Cl
N
2000
0 0
200
400
600
800
Binding energy (eV) 10000
(c)
O
8000
6000
Intensity (cps)
O
Intensity (cps)
CH2 C OCH2CH3
O
Cu C
4000
N
Si
2000
0
CH2 C
OH 0
O
3.4.
259
XPS Results
The XPS survey spectra for the silica-immobilized diethyliminodiacetate, iminodiacetic acid ligand systems and its copper(II) complex are given in Fig. 2. The spectrum for the immobilized diethyliminodiacetae
200
400
600
800
1000
1200
Binding energy (eV)
Figure 2. XPS survey spectra of (a) S-DIDA. (b) S-IDA. (c) Copper complex.
(S-DIDA) shows a weak intensity signal near 400 eV due to the amine nitrogen atoms and a weak doublet at 618.6 and 630.8 eV due to the iodine atoms. When the ethyl acetate undergoes hydrolysis there is a weak
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El-Nahhal et al.
form. The signals are fitted with two components centered at 399.5 and 401.8 eV assigned to free and amine cation, respectively. The latter is even clearly visible in the acetate form though its extent levels off at 20%. This could be due to a strong acid-base interaction with the silanol groups resulting in proton transfer towards some amine sites. In the case of the carboxylic form (Fig. 4(b)), there is a contribution of 50.7% of the protonated form. This may indicate that not all the diethyiminodiacetate groups are hydrolyzed by the HCl and the two forms are present as given below:
4500 4000
Intensity (cps)
3500 3000 2500 2000 1500 1000 500 278
280
282
284
286
288
290
292
294
296
CH2COOH
Binding energy (eV)
Figure 3.
N
C1s XPS peak spectrum fitting of S-DIDA.
CH2COOH
signal of the chlorine atoms and a slight but detectable decrease in the C1s relative intensity due to the hydrolysis of the ethoxy groups (Fig. 2(b)). In the case of the copper complex (Fig. 2(c)) the copper is detected by its Cu2p doublet located at ca. 933–953 eV binding energy range. Figure 3 displays C1s region for the S-DIDA ligand system fitted with four main components centered at 284.4, 285.1, 286.7 and 289.1 eV, which assigned to the C Si, C C/C H, C N/C O and COOR, respectively. The C1s components due to C O and C N bonds are lumped together because the chemical shift (0.5–0.8 eV) is small compared to the full width at half maximum (around 2 eV in the actual conditions). Figure 4 exhibits N1s region spectra from S-DIDA ligand system and its corresponding carboxylic acid
NH CH2COOH
49.3 %
50.7 %
In the case of the copper complex form, the N1s is similarly fitted with two peaks but the amine cation has a contribution of 25%. The decrease of the amine cation from 50% for the immobilized ligand S-IDA to about 25% in the complex, is probably due to complexation to the copper ions. This change in the nitrogen ratio forms provides evidence that the nitrogen atoms are involved in the coordination to the metal ions. Figure 5 depicts the Cu2p signal of the copper complex, with the Cu2p3/2 peak centered at 932.8 eV. The Cu(II) state is slightly evidenced by the existence of a shake-up satellite around 945 eV. The Cu2p3/2 binding energy, though slightly lower than expected, is
(a)
3800
CH2COO +
3500
(b)
3400 3600
Intensity (cps)
Intensity (cps)
3300 3400
3200
3000
3200 3100 3000 2900
2800 2800 2600
2700 390
395
400
405
410
Binding energy (eV)
Figure 4.
N1s XPS peak spectrum fitting of (a) S-DIDA. (b) S-IDA.
390
395
400
Binding energy (eV)
405
410
Immobilized Iminodiacetic Acid-Modified Silica
Table 3. XPS data of the diethyliminodiactate and iminodiacetic acid immobilized ligand systems and its cupper(II) complex. Materials
C
O
Si
N
N+
Cu
Core level BE (eV)
C1s (285)
O1s (532)
Si2p (102)
N1s (399.5)
N1s (401.7)
Cu2p3/2 (932.8)
S-DIDA
38.75
41.42
16.98
2.28
0.57
S-IDA
32.45
44.88
20.21
1.21
1.25
Complex
33.68
43.59
18.42
2.22
0.76
levels of all these modified silica networks are comparable. The uptake of copper ions by the S-IDA ligand system can be measured by Cu/N ratio which equals 45% for the copper complex. But when only the carboxylic acid form of the ligand (S-IDA) is considered for coordination then 1:1 metal to ligand complex is expected.
1.34
3.5. 8500
8000
Cu2p3/2
Cu2p1/2
Intensity (cps)
7500
7000 shake-up
6500
6000
5500
920
930
940
950
960
970
980
Binding energy (eV)
Figure 5.
261
Cu2p XPS spectrum.
consistent with some values reported in the literature for copper complexes such as Cu(OAc)2 [29]. Table 3 reports the atomic percentages of carbon, silicon, oxygen, nitrogen and iodine and their corresponding core-line binding energies (BE’s). Iodine, detected at the surface of S-DIDA has not been considered because it had a negligibly low concentration (I/C atomic ratio is 0.026). Iodine was straightforwardly detected due to the very high photoionization cross-section of I3d core level electron. Nevertheless, the detection of iodine provides an evidence for the presence of unreacted iodine atoms that are not accessible for replacement by the diethyliminodiacetate groups. Table 3 quantitatively confirms the increase in the carbon content in the order S-IDA < S-DIDA since S-DIDA has ester and not COOH end groups. The N/Si ratio can be used as a measure of the functionalisation of the various modified silicas. N/Si = 16.8, 12.2 and 16.2% for S-DIDA, S-IDA and copper complex respectively. Clearly, if one takes into account the experimental errors, the functionalisation
Thermal Analysis
Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) were examined for the free immobilized iminodiacetate ligand system and its nickel(II) complex. The thermogram of the immobilized iminodiacetic acid ligand system shows two DTG peaks. The first peak at 74.6◦ C where the ligand system loss 7.7% of its initial weight. This is attributed to loss of physisorbed water and alcohol from the pore system [30–32]. The second peak at 200◦ C is due to further loss of 4.7% weight of the polymer. That is probably due to cleavage and degradation of the organofunctional group bound to the silicon atoms [30–32] producing gases e.g. CO2 which takes place at 180◦ C as a product of the thermal composition [30–32]. The DTG curve shows a broad region at temperature range from 300–600◦ C, due to condensation of hydroxyl groups, left in the polymer to form siloxane bonds (dehydroxylation) [30–32]. The thermogram of the nickel(II) complex shows two characteristic peaks at ca. 75◦ C and 360◦ C. The second peak was observed at higher temperature than that of the free immobilized iminodiacetic ligand. Therefore the degradation of the functionalized ligand groups in the complex occurs at higher temperature than that of the free ligand. This suggests that the immobilized ligand system become more thermal stable on complexation with metal ions.
3.6.
FTIR Spectra
The FTIR spectra of the 3-iodopropyl, diethyliminodiacetate and iminodiacetatic acid modified silica show three regions of absorptions at 3500–3000 cm−1 , 1750– 1600 cm−1 and 1200–900 cm−1 due to ν(OH), ν(C O) and ν(Si O) respectively. Upon treatment with HCl the absorption band due to ν(C O) at 1740 cm−1 for the immobilized diethyliminodiacetate is shifted to 1670 cm−1 due to ν(C O) of the free acid form with the presence of small absorption at 1740 cm−1 even
262
El-Nahhal et al.
Table 4. Proton uptake capacity of the immobilized diethyliminodiacetate ligand system (S-DIDA) compared with elemental analysis data for nitrogen. mmol H+ /g ligand
Elemental analysis
0.5 h
1h
2h
4h
8h
24 h
48 h
72 h
mmol N/g
1.09
1.18
1.28
1.31
1.35
1.36
1.35
1.34
1.22
after the hydrolysis reaction. This indicates that some ethoxy groups remain unhydrolyzed.
uptake capacities by the immobilized iminodiacetate ligand system (S-IDA) as mmol Mn+ /g ligand are given below:
O
O -C-OEt
HCl
Metal ion
-C-OH
Maximum uptake
3.7.
Fe3+
Co2+
Ni2+
Cu2+
Zn2+
0.90
1.20
1.00
1.10
1.23
0.63
Proton Uptake Capacity
The proton uptake (mmol H+ /g ligand) was studied for immobilized diethyliminodiacetate ligand system (S-DIDA). The procedure was carried out by shaking 250 mg of the ligand with aqueous HCl solution (0.1 M) at different time intervals. Comparison between the capacity results and the calculated amount of nitrogen based on elemental analysis are given in Table 4. From Table 4 it is obvious that the proton uptake increases with time. It is clear that nearly 100% of protons were taken by the modified silica ligand system occurred within one hour. It is found that the mmol of protons taken by the modified silica ligand system at its maximum value is more than that obtained from nitrogen analysis (1.22 mmol N/g ligand). This could be explained due to the hydrolysis of the ethoxy groups of the functionalized ligand and the acidic hydrolysis of siloxane linkages (Si O Si), and therefore consuming a significant amount of protons as given below: O OH Cl O
Si
O
CH2
C
OC2H5
CH2
C OC2H5
N
O
3.8.
Mn2+
Metal Uptake
The metal ion uptake capacity (Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ) as mmol Mn+ /g ligand, was determined by shaking the functionalized ligand systems (S-IDA) with buffered solutions. The maximum metal
From the elemental analysis of N of the immobilized ligand (S-IDA) which is found (1.40%; 1.0 mmol N/g ligand) and the maximum metal ion uptake, it is possible to suggest 1:1 metal to ligand complexes are to be expected for all metal ions except for the zinc ions. It is clear that uptake of metal ions increases in the order: Cu2+ > Ni2+ > Co2+ > Mn2+ > Zn2+ . This is in consistent with the Irving William series of the formation constant [33]. The immobilized iminodiacetic acid ligand system reacts with aqueous solution of metal(II) chlorides producing metal chelate complexes in which the acetate oxygen and nitrogen atoms may involved in coordination to the metal ion. At pH 5–7, the iminodiacetic acid has the form of Zwitter ion that readily forms stable complexes when treated with metal(II) chlorides. 3.8.1. Effect of Shaking Time. The metal ion uptake capacities at different time intervals were examined for all metal ions. Figure 6 illustrate the uptake capacity of copper ion as example. The main results show that, the metal uptake is finished after 24 h and the maximum metal uptake is found at pH 5.5. Similar results have been found for the other metal ions (Mn2+ , Fe3+ , Co2+ , Ni2+ and Zn2+ ). 3.8.2. Effect of pH. The effect of the pH value on the uptake of Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ions by S-IDA is shown in Fig. 7. The results show an increase of metal ion uptake with increasing pH value and reached its maximum at pH 5.5. Only minor uptake capacity occurs at lower pH values (pH 1–3)
Immobilized Iminodiacetic Acid-Modified Silica
263
are inversely proportional to the particle size. This is expected as the surface area exposed to the metal ion increases by decreasing the particle size. Therefore, the ligand groups are readily accessible for complexation when contact with the metal ion solution. Maximum uptake was found for the shaking fine particle size material (mesh = 230). Low metal uptake capacity was found for the powdered material (mesh > 230) as it suffers from leaching as a result of mechanical shaking. Figure 6. Uptake of copper ions by S-IDA versus time for (a) Manganese, (b) Iron, (c) Cobalt, (d) Nickel, (e) Copper, and (f) Zinc.
3.8.4. Effect of Shaking. A comparison between the metal ion uptake using shaking and unshaked samples was performed. The results are given in Table 5. In general shaking samples exhibit higher metal ion uptake than unshaked samples, except for the fine particle size (mesh = 230). This could be due to a better diffusion of the metal ion with shaking, so the ligand groups become more accessible for the metal ions. Similar trends were reported for monoamine and diamine ligand systems [5]. 3.9.
Figure 7. Uptake of metal ions by S-IDA versus pH values (72 h shaking time).
due to protonation of amine group and undissociated carboxylic groups [5–11].
Stability Studies of S-IDA
The stability of the immobilized iminodiacetic acid ligand system S-IDA was examined upon treatment with buffer and metal ion aqueous solutions at different pH values.
3.8.3. Effect of Particle Size. The effect of the particle size of the functionalized ligand system (S-IDA) on the metal uptake was examined. The results are shown in Fig. 8. It is found that the metal uptake capacities
3.9.1. Effect of pH. Gravimetric analysis for the iminodiacetic acid ligand system upon treatment with buffer solutions at different pH values (1–13) is shown in Fig. 9. It is found that the leaching percentage of S-IDA is more significant in acid and base solutions. The ligand was completely dissolved at pH 13. The
Figure 8. Uptake of metal ions by S-IDA versus particle size (pH 5.5, 72 h shaking time).
Figure 9. Leaching percentage of S-IDA versus pH values (72 h shaking time).
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El-Nahhal et al.
Table 5.
Metal uptake by immobilized iminodiacetate ligand system with and without shaking (72 h). mmol Mn+ /g ligand (pH 5.5)∗ Cu2+
Particle size Coarse (mesh = 60)
Fe2+
Ni2+
Co2+
Mn2+
Zn2+
a
b
a
b
a
b
a
b
a
b
a
b
1.348
1.213
0.983
0.952
0.912
0.821
0.743
0.712
0.686
0.641
0.565
0.472
Medium (mesh = 120)
1.491
1.343
1.155
1.113
1.103
1.023
0.924
0.875
0.862
0.803
0.651
0.621
Fine (mesh = 230)
1.576
1.471
1.305
1.241
1.243
1.171
1.026
1.012
0.941
0.923
0.725
0.717
Powder (mesh > 230)
1.545
1.586
1.266
1.382
1.205
1.291
0.995
1.125
0.912
1.031
0.709
0.812
a: with shaking, b: without shaking. ∗ Mean three determinations with a maximum standard deviation of 0.05.
high degradation in basic medium is probably due to hydrolysis of Si O Si linkages, which is strongly catalyzed in the presence of OH− .
Table 6. Leaching percentages of S-IDA as a function of metal ion solutions. Mn+ Solution % Leaching
3.9.2. Effect of Shaking and Particle Size. Studying of leaching % as a function of particle size and shaking was performed at pH 5.5 using acetate buffer. It is found that leaching percentage of the material is inversely proportional to its particle size (Fig. 10). Degradation of the material is high in case of small particle size, as the specific surface of the sample increases. Mechanical shaking also increases the leaching of the small species from the siloxane network. 3.9.3. Effect of Nature of Metal Ion Solution. The stability of the iminodiacetate ligand system (S-IDA) was examined in different solutions of metal ions (Fe3+ , Mn2+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ). The gravimetric results are shown in Table 6. It is clear that the amount of species leached from the polymeric matrix varies
Figure 10. Leaching percentage versus particle size by S-IDA (pH 5.5) with and without shaking.
Mn2+
Fe3+
Co2+
Ni2+
Cu2+
Zn2+
F
4.0
5.4
4.2
4.3
4.6
3.7
2.3
from one metal to another in the following order. Fe3+ > Cu2+ > Ni2+ > Co2+ > Mn2+ > Zn2+ > F where F is a metal free solution buffered at pH 5.5. This is probably due to the presence of metal ions attacking polymeric matrix and so facilitate degradation and hydrolysis of Si O Si links [16]. Similar observations are also reported for monoamine ligand systems [16, 30]. The leaching is increased is probably affected by the pH of the metal ion solution. Conclusion The immobilized diethyliminodiactate ligand system was prepared by grafting diethyliminodiacetate on a silica functionalized with 3-idopropyl groups. Diethyliminodiacetate ligand system was then hydrolyzed to produce iminodiacetic acid ligand system. 13 C solid state NMR and XPS analysis indicated that not all iodine atoms are replaced by diethyliminodiactate. They also showed that not all ethylacetate groups are hydrolyzed upon treatment with HCl and therefore both the ester and acid forms exist. This immobilized ligand system exhibits high potential for the extraction of various diand trivalent metal ions such as (Fe3+ , Mn2+ , Co2+ , Ni2+ , Cu2+ and Zn2+ ). XPS, elemental and atomic absorption analysis reveals that 1:1 metal to ligand-metal complexes are accomplished.
Immobilized Iminodiacetic Acid-Modified Silica
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