- freely available
Molecules 2014, 19(1), 247-262; doi:10.3390/molecules19010247
Published: 24 December 2013
Abstract: A new chelating matrix, SiNP, has been prepared by immobilizing 1.5-dimethyl-1H-pyrazole-3-carbaldehyde on silica gel modified with 3-aminopropyl-trimethoxysilane. This new chelating material was well characterized by elemental analysis, FT-IR spectroscopy, cross polarization magic angle spinning solid state 13C-NMR, nitrogen adsorption-desorption isotherm, BET surface area, BJH pore size, and scanning electron microscopy (SEM). The new product exhibits good chemical and thermal stability as determined by thermogravimetry curves (TGA). The new prepared material was used as an adsorbent for the solid-phase extraction (SPE) of Pb(II), Cd(II), Cu(II) and Zn(II) from aqueous solutions using a batch method, prior to their determination by flame atomic adsorption spectrometry. The adsorption capacity was investigated using kinetics and pH effects. Common coexisting ions did not interfere with separation and determination.
Environment pollution by heavy metals has caused lately much concern because of their general and specific toxicities. The most toxic heavy metals, namely lead, cadmium, copper and zinc, can be distinguished from other pollutants, because they cannot be degraded naturally, but rather accumulate in living organisms. Therefore they cause different diseases and disorders, even at low concentrations [1,2,3,4,5,6]. Therefore, determination of heavy metals in environmental and biological materials is an important screening procedure in environmental pollution and occupational exposure studies.
Traditionally, extraction is carried out liquid-liquid extraction, co-precipitation, and ion exchange, etc. These methods have non-economic disadvantages. They often require large amount of high purity organic solvents, some of which are themselves harmful to health and cause environmental problems. Nowadays, several methods are used for pretreatment of the samples. Solid phase extraction (SPE) [7,8,9,10,11,12], has commonly been used as a technique for pre-concentration/separation of various inorganic and organic species. SPE has several major advantages that include higher enrichment factors, simple operation, safety with respect to hazardous samples, high selectivity, lower cost and less time, the ability to combine it with different modern detection techniques .
A variety of ligands or functional groups are immobilized onto a solid support matrix as a solid phase extractant for the purpose of extraction and enrichment of trace metal ions from environmental samples. Silica gel is of great importance as a solid support because it possesses some definite advantages . The silica support is chosen for its high surface area, high mechanical and thermal stability. In addition, it is easily modified , by reacting with organofuctionalized silanes through its surface silanol groups. These covalently bonded organic groups are highly stable and resistant to removal from the surface by organic solvents or water . To this end, a great number of organic molecules were immobilized on silica gel surface, xylenol orange , 2-thiophenecarboxaldehyde , di(n-propyl)thiuram disulfide , 4-acylpyrazolone , aminothioamidoanthraquinone , 1,8-dihydroxyanthraquinone , murexide , oxime derivatives , resacetophenone , diphenyldiketone monothiosemicarbazone . These systems can be operated indefinitely without loss of the expensive organic molecules. Their potential applications are attributable essentially to the nature of the grafted ligands. Indeed, the most commonly attached molecules have chelating ability due to their donor atoms, such as oxygen, nitrogen and sulphur, which have a large capability to form complexes with a series of metal ions, leading in some cases, to distinguishable selective extraction properties.
In this context, for many years, the ability of pyrazole and its derivatives to act as ligands with sp2 hybrid nitrogen donors have been the research subjects of many coordination chemists. This is evident from the large number of articles on this topic, several of them being reviews [26,27,28]. In continuation of our work in this field [29,30,31,32], this paper describes the synthesis and the characterization of a new material obtained by grafting onto porous silica functionalized compounds which can act as in a N,N'-bidentate fashion [33,34] forming five membered chelating rings. The immobilization of this ligand on silica gel was carried out with a long arm spacer in order to facilitate the contact between the receiver and the metal ion. The new chelating material was well characterized and its adsorption capacities towards highly toxic heavy metals ions such as Pb(II), Cd(II), Cu(II) and Zn(II) was investigated and the extracted amounts of metals ions were determined by atomic absorption measurements. This new material presents high adsorption of lead compared to the other tested metals ions.
2. Results and Discussion
2.1. Linker Synthesis
The synthetic procedure for the new chelating material is summarized in Scheme 1. The preparation involves the reaction of activated silica gel with 3-aminopropyltrimethoxysilane in toluene to install amino groups attached to the silica surface . These NH2-groups onto the silica surface were then reacted with 1,5-dimethyl-1H-pyrazole-3-carbaldehyde under mild conditions (reflux, atmospheric pressure and 8 h) using anhydrous ethanol as solvent, to form the new chelating sorbent SiNP.
2.2.1. Elemental Analysis
The elemental analysis of carbon and nitrogen (not present in the starting activated silica) of aminopropylsilica SiNH2 makes it possible to characterize and highlight the introduced organic group on the silica surface. The microanalysis results (%C = 4.46, %N = 1.66 and %H = 1.27) suggests that two methoxy groups were substituted by silanol. The final SiNP-Schiff base material showed also an increase in the percentage of C, N and H (%C = 5.32, %N = 1.90 and %H = 1.34), which means that the pyrazole unit was immobilized on the silica gel surface.
2.2.2. FT-IR Characterization
To confirm the presence of functional groups in the material, FT-IR spectra were performed for free silica gel, SiNH2 and SiNP materials (Figure 1). The sharp features around 1,100 cm−1 indicated Si-O-Si stretching vibrations. The presence of adsorption water was reflected by ν(OH) vibration around 3,446 and 1,620 cm−1. The bonds around 970 cm−1 resulted from Si-O vibration . Compared to free silica gel, the spectrum of SiNH2 exhibits some new peaks such as the CH2 vibration band at 2,691 cm−1 and the NH2 vibration at 1,560 cm−1 [37,38]. The characteristic features of SiNP compared with SiNH2 were the disappearance of the adsorption band at 1,560 cm−1 due to the reaction of the primary amine (-NH2) and the appearance of a new characteristic bond around 1,500 cm−1 resulting from C=N and C=N vibrations, which confirms the anchoring of the organic molecule onto the silica surface.
2.2.3. Scanning Electron Micrographs
Scanning electron micrographs (SEM) were obtained on the free silica and chemically modified silicas in order to detect differences in their surfaces. SEM of silica gel, SiNH2 and SiNP in Figure 2 were obtained at 300× and 1,200× magnification. The SEM was displayed to clarify the un-agglomeration of the silica gel particles after treatment to support the claiming of regular distribution of the functional group on the whole surface. It was evident that the loaded functional groups were distributed on the whole surface that made the surface of the product SiNP become rough.
2.2.4.TGA Analysis and Thermal Stability
The thermogravimetric curves for all surfaces enable the establishing of information on thermal stability and also to confirm the amount of the compounds immobilized, as shown in Figure 3. The profile indicates a degradation process between 146 and 800 °C which confirms the high thermal stability for the prepared material.
The free silica presents a first mass loss stage of 3.15% in the interval from room temperature to 110 °C, assigned to physically adsorbed water and a second loss of 5.85% from 110 to 800 °C assigned to condensation of the free silanol groups which causes siloxane bond formation (Si-O-Si) [39,40]. Again two distinct mass loss steps were detected for the SiNH2 sample. The first one, a small mass loss of 1.56% in the room temperature to 100 °C range is attributed to the remaining silanol hydration water, as a consequence of the use of these groups in the immobilization process. On the other hand, a pronounced mass loss increase of 9.77% was observed for the second step, between 208 and 800 °C, which corresponds to the organic matter added onto the surface during immobilization. The final SiNP material presented two distinct mass loss stages. Following the preceding interpretation, the first mass loss of 2.27% in the 25–102 °C range is assigned to adsorbed water, and other mass loss of 12.49% between 231.47 and 800 °C is attributed to the decomposition of the pyrazole fraction immobilized on the surface of silica gel, together with the condensation of the remaining silanol groups. The pronounced increase in mass loss reflects the higher amount of the anchored organic groups.
2.2.5. 13C-NMR Characterization
Important features related to the immobilization of pendant groups on the inorganic structure of the formed hybrid can be obtained through solid state 13C-NMR spectroscopy, as shown in Figure 4. The signals observed for 3-aminopropyl-silica SiNH2 at δ = 9.02, 24.79 and 42.62 ppm have been assigned to the propyl carbon Si-CH2, -CH2- and N-CH2, respectively. The signal at 50.62 ppm was assigned to the unsubstituted methoxy group as confirmed by microanalysis.
2.2.6. Chemical Stability
Chemical stability of the newly synthesized material SiNP was examined in various acidic and buffer solutions (pH 1–7). No change in the material structure was observed even after 24 h of contact. The high stability exhibited by the attached organofunctional group is presumably due to the pendant group. It has been shown that when the length of the hydrocarbon bridge was more than two methylene groups, the rupture of Si–C bond did not occur in a mineral acid medium, due to the length of the chain; longer chains no longer have a functional handle that can undergo β-elimination of the Si cation [41,42].
2.2.7. Surface Properties
To show the porosity changes of the silica induced by the introduction of 3-aminopropyl and pyrazole unit, we measured the surface area SBET (Brunauer–Emmett–Teller), pore volumes, and pore diameters of both silica and its derivatives with nitrogen adsorption–desorption isotherms (Figure 5) and by Barrett–Joyner–Halenda (BJH) pore diameters methods [43,44]. The density of the pendant groups covalently attached to the inorganic silica backbone changes the original characteristics of the surface. As shown in Table 1, the initial specific surface area SBET of 305.21 m2g−1 and a pore volume of 0.77 cm3g−1, decreases as the immobilization takes place to give 283.08 m2g−1 and a pore volume of 0.69 cm3g−1. A decrease in SBET is mainly due to the presence of the organic moieties that can block the access nitrogen to the silica base. On the other hand, we observed that SiNP has an additional BET surface area decrease as additional group immobilization takes place to give 236.60 m2g−1, and a pore volume of 0.64 cm3g−1. The decreased surface area and pore volume in SiNP are attributable to the grafted 1.5-dimethyl-1H-pyrazole-3-carbaldehyde.
|Table 1. Physical properties of silica derivatives.|
|Silica derivatives||Specific surface SBET (m2 g−1)||Pore volume (cm3 g−1)|
Moreover, the nitrogen adsorption–desorption isotherm for silica derivatives, shown in Figure 5, are type IV according to the IUPAC classification and display a pronounced hysteresis for partial pressures P/P0 > 0.4.
2.3. Solid–Liquid Adsorption of Metal Ions by SiNP
The effects of pH and shaking time on the extraction of the three metal ions were studied by the batch method. The modified silica gel (10 mg) was equilibrated by shaking with 10 mL of a solution containing different concentrations of metal ions (243.01 mg/L for Pb(II), 102.93 mg/L for Cd(II), 75.63 mg/L for Cu(II) and 74.02 mg/L for Zn(II)), for different time intervals (1, 15, 30 min and 1, 1.5, 2, 3, 4, 5, 6, and 24 h) and different pH values (1–8). The metal ions were in excess over the sorption capacity. The concentration of metal ions was determined by means of atomic absorption measurements. The amount of metal ions adsorbed by the synthesized material SiNPz from aqueous solution was calculated using the following equations :
2.3.1. Effect of pH
It is well-known that binding of metal ions to the chelate compounds either in solution or loaded on solid supports is mainly dependent on several factors such as the nature, charge and size of the metal ions [46,47], nature of the donor atoms and their binding characteristics [48,49], and the buffering conditions. These factors are very well documented in solution chemistry as well in solid-phase extraction of certain metals by organic chelates immobilized on the surface of solid supports such as silica gel, nanomaterials or polymeric species. Therefore, to evaluate the suitability of the newly synthesized SiNP for metal ions extraction and binding, we studied the effect of pH of the metal ion solution on the metal capacity values as one of the most significant controlling factors in such a process.
The adsorption properties of SiNP were investigated in the pH 1–8 range as shown in Figure 6. Results reveal that the metal ion uptake of the adsorbent varies significantly as the pH changes. At lower pH values, the retention of metal ions by the functionalized silica SiNP is not significant since the ligand must be almost entirely in its protonated form. As the pH increases, the protonation becomes weak, which enhances the chelation and adsorption of metal ions. At pH > 8, the retention of metal ions decreased because of the hydrolysis of metal ions (leading to the hydroxides of M(II): M(OH)+ and M(OH)2), this makes it difficult to distinguish between the hydrolyzed or adsorbed M(II). Therefore, the optimum pH for the maximum sorption of Cu(II) was at pH ≥ 5, Cd(II) and Zn(II) at pH ≥ 6 and Pb(II) at pH ≥ 7. Data are given in Table 2.
|Table 2. Metal ion uptake of SiNP (Qw, mg/g) according to pH.|
2.3.2. Effect of Stirring Time
The stirring time used for the adsorption of the metal ion by the modified silica gel and the attainment of equilibrium conditions is of considerable importance. Effect of stirring time on the adsorption of Pb(II), Cd(II), Cu(II) and Zn(II) by SiNP was studied by batch experiments. As can be seen from Figure 7, the kinetic curves of Pb(II), Cd(II), Cu(II) and Zn(II) showed that the adsorption was rapid and the plateau was reached after about 30 min of contact. The rapid adsorption of different metal ions suggests that the two nitrogen active donor atoms on the modified silica gel surface are oriented in such a way that their accessibility is not hindered and consequently, fast interaction with the free metal ions present in solution is feasible. Indeed, the two nitrogens (of the grafted pyrazole and of the imine) act as a convergent chelating bidentate donor. The term convergent refers to the nitrogen donor atoms coordinating to the same metal center, thus leading to a five-membered ring which is part of several such rings when the whole ligand is considered. It is well known that five-membered ring chelates are more stable than six-membered and four-membered ones . The rapid kinetics have a significant practical importance, as it will facilitate smaller reactor volumes ensuring efficiency and economy.
The variation in sorption capacities of various metal ions probably arises due to their size, degree of hydration, and binding constants of their complexes with the matrix.
2.3.3. Effect of Coexisting Ions
It is well know that metal cations and acyclic pyrazolic compounds do not complex alkali metal cations at all, while the macrocyclic pyrazolic ligands form complexes both transition and alkali metals [51,52,53]. Thus, the effects of common coexisting ions in water samples on the recovery of each metal were also studied. In these experiments, 50 mL of a solution containing 0.1 μg/mL of a metal ion and various amounts of interfering ions were treated according to the recommended procedure. An ion was considered to interfere when its presence produced a variation in the extraction recovery of sample more than ±5%. The results show that in excess of 10,000-fold, the Li+, K+, Na+, Ca2+ and Mg2+ ions show no significant interferences in the extraction and determination of each Pb(II), Cd(II), Cu(II) and Zn(II) metals. As can be seen, SiNP has a high tolerance limit for alkali and alkaline earth metals. This is particularly useful for the analysis of transition elements of group 12 and Pb(II), in natural water samples, for example seawater, which contains large amounts of alkali and alkaline earth metal ions.
2.3.4. Comparison with Alternative Materials
Table 3 shows the adsorption of Pb(II), by other material reported in the literature. It is clear that the functionalized silica described in this work presents further improvement and shows better values and higher affinity for the effective adsorption for Pb(II) and other metals under study.
|Table 3. Comparison of SiNP with other reported sorbents for Pb(II) absorption.|
|Support: silica gel/ligand||Reference||Capacity (mg of Pb2+/g of silica)|
|Pyrazol-3-ylimine (this work)||-||74.89|
|Acid red 88||||03.35|
3.1. General Information
All solvents and other chemicals (purity > 99.5%, Aldrich, Saint-Louis, MO, USA) were of analytical grade and used without further purification. Silica gel (E. Merck, Darmstadt, Germany) with particle size in the range of 70–230 mesh, median pore diameter 60 Å, was activated before use by heating it at 160 °C during 24 h. The silylating agent 3-aminopropyltrimethoxtsilane (Janssen Chimica, Geel, Belgium) was used without purification. All metal ions were determined by atomic adsorption measurements were performed by a Spectra Varian A.A. 400 spectrophotometer (Oujda, Morocco). The pH value was controlled by a pH 2006, J. P. Selecta s. a. (Barcelona, Span); Elemental analyses were performed by the Microanalysis Centre Service (CNRS, Lille, France). FT-IR spectra were obtained with a Perkin Elmer System 2000 instrument (Oujda, Morocco). SEM image were obtained on an FEI-Quanta 200 (Lille, France). The mass loss determinations were performed in 90:10 oxygen/nitrogen atmospheres on a Perkin Elmer Diamond TG/DTA, at a heating rate of 10 °C min−1 (Blois, France). The 13C-NMR spectrum of the solid state was obtained with a CP MAX CXP 300 MHz instrument (Lille, France). The specific area of the modified silica was determined by using the BET equation. The nitrogen adsorption-desorption was obtained by means of a Thermoquest Sorpsomatic 1990 analyzer (Lille, France), after the material had been purged in a stream of dry nitrogen. Molecular weights were determined on a JEOL JMS DX-300 Mass Spectrometer (CNRST, Rabat, Morocco).
3.2. Synthesis of 3-Aminopropylsilica (SiNH2)
The first stage in the preparation was the reaction between the silylating agent and silanol groups on the silica surface. Activated silica gel SiO2 (25 g) suspended in dried toluene (150 mL) was refluxed and mechanically stirred under nitrogen atmosphere for 2 h. To this suspension, aminopropyltrimethoxysilane (10 mL) was added dropwise and the mixture was kept under reflux for 24 h. The solid was filtered, washed with toluene and ethanol. It was then Soxhlet extracted with a 1:1 mixture of ethanol and dichloromethane for 12 h, to remove the silylating reagent residue. The immobilized silica gel, named SiNH2, was dried under vacuum at room temperature.
3.3. Synthesis of 1.5-Dimethyl-1H-pyrazole-3-carbaldehyde
(1.5-Dimethyl-1H-pyrazol-3-yl) methanol [62,63] (1.2 g, 9.52 mmol) was dissolved in 1,4-dioxane (100 mL). Activated manganese dioxide (21.88 g) was added to the solution and the suspension was stirred under reflux for 5 h. The reaction was monitored by TLC (alumina, CH2Cl2 as eluent). The hot suspension was filtered and the solid MnO2 was washed with boiling 1,4-dioxane. The solvent was removed on a rotary evaporator to give the expected product as a liquid (0.7 g, yield 59.23%). Rf = 47.51). 1H-NMR (CDCl3) δ ppm: 2.13 (s, 3H, C-CH3), 3.84 (s, 3H, N-CH3), 6.51 (s, 1H, Pz-H), 9.84 (s, 1H, C-H, aldehyde). 13C-NMR (CDCl3) δ ppm: 11.20(1C, C-CH3), 67.05 (1C, N-CH3), 105.19 (1C, CH3-C=C-), 140.88 (1C, C-CH3), 149.19 (1C, N=C-), 186.40 (1C, C-H, aldehyde). IR ν(C=O) = 1691cm−1. MS m/z = 124 (M+).
3.4. Synthesis of ((1,5-Dimethyl-1H-pyrazol-3-yl)methylene)imine-Substituted Silica (SiNP)
For the synthesis of SiNP, a mixture of 3-aminopropylsilica (SiNH2, 5 g) and 1.5-dimethyl-1H-pyrazole-3-carbaldehyde (3 g) in dry ethanol (60 mL) was stirred at reflux for 8 h. After being filtered, the solid product was Soxhlet extracted with acetonitrile, methanol and dichloromethane for 12 h, respectively. The product was then dried under vacuum at 70 °C over 24 h.
3.5. Batch Experiments
The effects of solution pH and contact time on the sorption of metal ions were evaluated on batch method. A suspension of adsorbent (SiNP, 10 mg) in metal solution (10 mL) containing different concentrations of metal ions (243.01 mg/L for Pb(II), 102.93 mg/L for Cd(II), 75.63 mg/L for Cu(II) and 74.02 mg/L for Zn(II)), was mechanically stirred at room temperature for 1 min to 24 h at 25 °C and under various pH conditions. The mixture was then filtered off and the amount of metal ion in the filtrate solution was determined by atomic adsorption measurement using standard solutions for calibration. Analyses were performed in duplicate for each sample and only the mean data are reported.
1.5-Dimethyl-1H-pyrazole-3-carbaldehyde was successfully bound on the silica surface after modification by 3-aminopyltrimethoxysilane. The structural, chemical and metal ion adsorption properties of this newly prepared sorbent was investigated. The adsorbent showed a high adsorption capacity towards Pb(II), Cd(II), Cu(II) and Zn(II) metal ions, with the best adsorption capacity of SiNP being 74.89 mg/g for Pb(II) ions. The new material was well characterized by elemental analysis, FT-IR spectra, 13C-NMR, nitrogen adsorption-desorption isotherm, BET surface area, B.J.H. pore sizes, thermogravimetry curves (TGA), and scanning electron microscopy (SEM). The new chelating surface exhibits good chemical and thermal stability. The sample was easily regenerated by soaking the sample in 6 N HCl for a few minutes (5–10 mL of 6 N HCl per g of support). The sorbent was regenerated five times, and showed no significant decrease in extraction percentage.
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project Number RGP-VPP-007. The authors gratefully acknowledge Bertrand REVEL, Research Engineer in the common center of NMR measurements of the University of Lille 1, for his assistance in recording the 13C-NMR spectra. We would also like to thank Ahmed MRABTI, technician at COSTE (Mohamed the 1st University) for the atomic absorption measurements.
Conflicts of Interest
The authors declare no conflict of interest.
- Babel, S.; Kurniawan, T.A. Low-cost adsorbents for heavy metal uptake from contaminated water: A review. J. Haz. Mater. 2003, 97, 219–243, doi:10.1016/S0304-3894(02)00263-7.
- Saeed, M.M.; Ahmed, M. Retention, kinetics and thermodynamics pro-file of cadmium adsorption from iodide medium onto polyurethane foam and its separation from zinc bulk. Anal. Chem. Acta 2004, 525, 289–297, doi:10.1016/j.aca.2004.08.011.
- Sanchez, L.R.B.; de La Riva, B.S.V.; Fernandez, J.M.C.; Pereiro, R.; Medel, A.S. Determination of lead and mercury in sea water by preconcentration in a flow injection system followed by atomic adsorption spectrometry detection. Talanta 2001, 55, 1071–1078, doi:10.1016/S0039-9140(01)00523-9.
- Silbergeld, E. The Elimination of Lead from Gasoline: Impacts of Lead in Gasoline on Human Health, and the Costs and Benefits of Elimination Lead Additives, Draft Paper; The World Bank: Washington, DC, USA, 1996; p. 3.
- Solé, E.; Ballbriga, A.; Dominguez, C. Lead exposure in children: Levels in blood, prevalence of intoxication and related factors. BioMetals 1998, 11, 189–197, doi:10.1023/A:1009253611000.
- Goswami, A.; Singh, A.K. Silica gel functionalized with resacetophenone: Synthesis of a new chelating matrix and its application as metal ion collector for their flame atomic absorption spectrometric determination. Anal. Chim. Acta 2002, 454, 229–240, doi:10.1016/S0003-2670(01)01552-5.
- Melek, E.; Tuzen, M.; Soylak, M. Flame atomic absorption spectrometric determination of cadmium(II) and lead(II) after their solid phase extraction as dibenzyldithiocarbamate chelates on Dowex Optipore V-493. Anal. Chim. Acta 2006, 578, 213–219, doi:10.1016/j.aca.2006.07.003.
- Jiang, N.; Chang, X.J.; Zheng, H.; He, Q.; Hu, Z. Selective solid-phase extraction of nickel (II) using a surface-imprinted silica gel sorbent. Anal. Chim. Acta 2006, 577, 225–231, doi:10.1016/j.aca.2006.06.049.
- Duan, T.C.; Song, X.J.; Jin, D.; Li, H.F.; Xu, J.W.; Chen, H.T. Preliminary results on the determination of ultratrace amounts of cadmium in tea samples using a flow injection on-line solid phase extraction separation and preconcentration technique to couple with a sequential injection hydride generation atomic fluorescence spectrometry. Talanta 2005, 67, 968–974, doi:10.1016/j.talanta.2005.04.017.
- Suvardhan, K.; Suresh Kumar, K.; Rekha, D.; Jayaraj, B.; Krishnamurthy, N.G.; Chiranjeevi, P. Preconcentration and solid-phase extraction of beryllium, lead, nickel, and bismuth from various water samples using 2-propylpiperidine-1-carbodithioate with flame atomic absorption spectrometry (FAAS). Talanta 2006, 68, 735–740, doi:10.1016/j.talanta.2005.05.020.
- Wan, Z.; Xu, Z.R.; Wang, J.H. Flow injection on-line solid phase extraction for ultra-trace lead screening with hydride generation atomic fluorescence spectrometry. Analyst 2006, 131, 141–147, doi:10.1039/b511829h.
- Fang, J.; Iang, Y.J.; Yan, X.P.; Ni, Z.M. Selective quantification of trace palladium in road dusts and roadside soils by displacement solid-phase extraction online coupled with electrothermal atomic absorption spectrometry. Environ. Sci. Technol. 2005, 39, 288–292, doi:10.1021/es049303+.
- Haginaka, J. Selectivty of affinity media in solide-phase extraction of analytes. Trends Anal. Chem. 2005, 24, 407–415, doi:10.1016/j.trac.2004.12.005.
- Sarkar, A.R.; Datta, P.K.; Sarkar, M. Sorption recovery of metal ions using silica gel modified with salicyladoxime. Talanta 1996, 43, 1857–1862, doi:10.1016/0039-9140(96)01953-4.
- Price, P.M.; Clark, J.H.; Macquarrie, D.J. Modified silicas for clean technology. J. Chem. Soc. Dalton Trans. 2000, 101–110, doi:10.1039/A905457J.
- Grigoropoulou, G.; Stathi, P.; Karakassides, M.A.; Louloudi, M.; Deligibiannakis, Y. Functionalized SiO2 with N-,S-containing ligands for Pb(II) and Cd(II) adsorption. Colloids Surf. 2008, 320, 25–35, doi:10.1016/j.colsurfa.2008.01.007.
- Fan, J.; Wu, C.; Peng, C.; Peng, P. Preparation of xylenol orange functionalized silica gel as a selective solid phase extractor and application for preconcentration-separtion of mercury from waters. J. Haz. Mater. 2007, 145, 323–330, doi:10.1016/j.jhazmat.2006.11.025.
- Jamali, M.R.; Assadi, Y.; Shemirani, F.; Salavati-Niasari, M. Application of thiophene-2-carbaldehyde-modified mesoporous silica as a new sorbent for separation and preconcentration of palladium prior to inductively coupled plasma atomic emission spectrometric determination. Talanta 2007, 71, 1524–1529, doi:10.1016/j.talanta.2006.07.034.
- Akhond, M.; Absalan, G.; Sheikhian, L.; Eskandari, M.M.; Sharghi, H. Di (n-propyl) thiuram disulfide bonded on silica gel as a new sorbent for separation, preconcentration, and measurement of silver ion from aqueous samples. Sep. Purif. Technol. 2006, 52, 53–59, doi:10.1016/j.seppur.2006.03.014.
- Amarasekara, A.S.; Owereh, O.S.; Aghara, S.K. Synthesis of 4-acylpyrazolone Schiff base ligand grafted silica and selectivity in adsorption of lanthanides from aqueous solutions. J. Rare Eart. 2009, 27, 870–874, doi:10.1016/S1002-0721(08)60352-X.
- Ngeontae, W.; Aeungmaitrepirom, W.; Tuntulani, T. Chemically modified silica gel with aminothioamidoanthraquinone for solid phase extraction and preconcentration of Pb(II), Cu(II), Ni(II), Co(II) and Cd(II). Talanta 2007, 71, 1075–1082, doi:10.1016/j.talanta.2006.05.094.
- Goswami, A.; Singh, A.K. 1,8-Dihydroxyanthraquinone anchored on silica gel: Synthesis and application as solid phase extraction for lead(II), Zinc(II) and cadmium(II) prior to determination by flame atomic adsorption spectrometry. Talanta 2002, 58, 669–678.
- Sadeghi, S.; Sheikhzadeh, E. Solid phase extraction using silica gel modified with murexide for preconcentration of uranium (VI) ions from water samples. J. Haz. Mater. 2009, 163, 861–868, doi:10.1016/j.jhazmat.2008.07.053.
- Gubbuk, I.H.; Hatay, I.; Coskun, A.; Ersoz, M. Immobilization of oxime derivative on silica for the preparation of new adsorbent. J. Haz. Mater. 2009, 172, 1532–1537, doi:10.1016/j.jhazmat.2009.08.024.
- Sharma, R.K.; Pandey, A.; Gulati, S.; Adholeya, A. An optimized procedure for preconcentration, determination and on-line recovery of palladium using highly selective diphenyldiketone-monothiosemicarbazone modified silica gel. J. Haz. Mater. 2012, 209, 285–292.
- Mukherjee, R. Coordination chemistry with pyrazol-based chelating ligands: Molecular structural aspects. Coord. Chem. Rev. 2000, 203, 151–218, doi:10.1016/S0010-8545(99)00144-7.
- Trofimenko, S. Recent advances in poly(pyrazolyl)borate (scorpionate) chemistry. Chem. Rev. 1993, 93, 943–980, doi:10.1021/cr00019a006.
- Trofimenho, S. The coordination chemistry of pyrazole-derived ligands. Prog. Inorg. Chem. 1986, 34, 115–210, doi:10.1002/9780470166352.ch3.
- Radi, S.; Toubi, Y.; Tighadouini, S.; Baquet, M. Solid-phase extraction of Hg(II), Zn(II) and Cd(II) from water using silica gel modified with bipyrazolic tripodal receptor. Ind. J. Chem. Tech. 2013, 20, 423–428.
- Radi, S.; Basbas, N.; Tighadouini, S.; Bacquet, M.; Degoutin, S.; Cazier, F. New amine-modified silica: Synthesis, characterisation and its use in the Cu(II)-Removal from aqueous solutions. Prog. Nanotech. Nanomat. 2013, 2, 108–116, doi:10.5963/PNN0204002.
- Radi, S.; Toubi, T.; Bacquet, M.; Degoutin, S.; Cazier, F. 1-(Pyridin-2-yl) Imine functionalized silica gel: Synthesis, Characterization and preliminary use in metal ion extraction. Sep. Sci. Tech. 2013, 48, 1349–1355, doi:10.1080/01496395.2012.726309.
- Radi, S.; Toubi, Y.; Bacquet, M. Synthesis of pyridin-3-yl-functionalized silica as a chelating sorbent for solid-phase adsorption of Hg(II), Pb(II), Zn(II) and Cd(II) from water. Chem. Res. Intermed. 2013, 39, 3791–3802, doi:10.1007/s11164-012-0881-6.
- Montoya, V.; Pons, J.; Garcia-Anton, J.; Solans, X.; Font-Bardia, M.; Ros, J. New (η3-Allyl) palladium complexes with pyridylpyrazole ligands: Synthesis, characterization, and study of the influence of N1 substituents on the apparent allyl rotation. Organometallics 2007, 26, 3183–3190, doi:10.1021/om070093u.
- Chang, S.-Y.; Chen, J.-L.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Jiang, C.-M.; Chou, P.-T. Blue-emitting platinum(II) complexes bearing both pyridylpyrazolate chelate and bridging pyrazolate ligands: Synthesis, structures and photophysical properties. Inorg. Chem. 2007, 46, 11202–11212, doi:10.1021/ic701586c.
- Qu, R.; Wang, M.; Sun, C.; Zhang, Y.; Ji, C.; Chen, H.; Meng, Y.; Yin, P. Chemical modification of silica-gel with hydroxyl- or amino-terminated polyamine for adsorption of Au(III). Appl. Surf. Sci. 2008, 255, 3361–3370.
- Han, D.M.; Fang, G.Z.; Yan, X.P. Preparation and evaluation of a molecularly imprinted sol–gel material for on-line solid-phase extraction coupled with high performance liquid chromatography for the determination of trace pentachlorophenol in water samples. J. Chromatogr. A 2005, 110, 131–136.
- Jiang, Y.J.; Gao, Q.M.; Yu, H.G.; Chen, Y.R.; Deng, F. Intensively competitive adsorption for heavy metal ions by PAMAM-SBA-15 and EDTA-PAMAM-SBA-15 inorganic-organic hybrid materials. Micropor. Mesopor. Mater. 2007, 103, 316–324, doi:10.1016/j.micromeso.2007.02.024.
- Abou-El-Sherbini, K.; Kenawy, I.; Hamed, M.; Issa, R.; Elmorsi, R. Separation and preconcentration in a batch mode of Cd(II), Cr(III, VI), Cu(II), Mn(II, VII) and Pb(II) by solid-phase extraction by using of silica modified with N-propylsalicylaldimine. Talanta 2002, 58, 289–300, doi:10.1016/S0039-9140(02)00248-5.
- Sales, J.A.A.; Airoldi, C. Calorimetric investigation of metal ion adsorption on 3-glycidoxypropyltrimethylsiloxane + propane-1,3-diamine immobilized on silica gel. Thermochim. Acta 2005, 427, 77–83, doi:10.1016/j.tca.2004.08.015.
- Sales, J.A.A.; Faria, F.P.; Prado, A.G.S.; Airoldi, C. Attachment of 2-aminomethyl-pyridine molecule onto grafted silica gel surface and its ability in chelating cations. Polyhedron 2004, 23, 719–725, doi:10.1016/j.poly.2003.11.051.
- Roumeliotis, P.; Kurganov, A.A.; Davankov, V.A. Effect of the hydrophobic spacer in bonded (Cu(l-hydroxyprolyl)alkyl)+ silicas on retention and enantioselectivity of α-amino acids in high-performance liquid chromatography. J. Chromatogr. A 1983, 266, 439–450, doi:10.1016/S0021-9673(01)90915-X.
- Kudryavtsev, G.V.; Milchenko, D.V.; Bernadyuk, S.Z.; Vertinskaya, T.E.; Lisichkin, G.V. Synthesis and properties of phosphate cation-exchangers based on silica. Theor. Exp. Chem. USSR 1988, 23, 658–663, doi:10.1007/BF00534608.
- Brunauer, S.; Emmett, P.H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309–319, doi:10.1021/ja01269a023.
- Banerjee, I.A.; Yu, L.; Matsui, H. Cu nanocrystal growth on peptide nanotubes by biomineralization: Size control of Cu nanocrystals by tuning peptide conformation. Proc. Natl. Acad. Sci. USA 2003, 100, 14678–14682, doi:10.1073/pnas.2433456100.
- Xue, X.; Li, F. Removal of Cu (II) from aqueous solution by adsorption onto functionalized SBA-16 mesoporous silica. Micropor. Mesopor. Mater. 2008, 116, 116–122, doi:10.1016/j.micromeso.2008.03.023.
- Pearson, R.G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85, 3533–3539, doi:10.1021/ja00905a001.
- Myers, R.T. Thermodynamics of chelation. Inorg. Chem. 1978, 17, 952–958.
- Mahmoud, R.E. Silica gel-immobilized Eriochrome blach-T as a potential solid phase extractor for zinc (II) and magnesium (II) from calcium (II). Talanta 1997, 45, 309–315, doi:10.1016/S0039-9140(97)00132-X.
- Maquieira, A.; Elmahadi, H.A.M.; Puchades, R. Immobilized cyanobacteria for online trace metal enrichment by flow injection atomic absorption spectrometry. Anal. Chem. 1994, 66, 3632–3638, doi:10.1021/ac00093a016.
- Lehn, J.M.; Sauvage, J.P. (2)-Cryptates: Stability and selectivity of alkali and alkaline-earth macrobicyclic complexes. J. Am. Chem. Soc. 1975, 97, 6700–6707, doi:10.1021/ja00856a018.
- Radi, S.; Yahyi, A.; Ramdani, A.; Zidane, I.; Hacht, B. A new tetrapyrazolic macrocycle. Synthesis and its use in extraction and transport of Na+, Li+, and K+. Tetrahedron 2006, 62, 9153–9155.
- Radi, S.; Ramdani, A.; Lekchiri, Y.; Morcellet, M.; Crini, G.; Morcellet, J.; Janus, L. New tetrapyrazolic macrocycle. Synthesis and cation binding properties. J. Chem. Res. 2003, 11, 712–714.
- Radi, S.; Ramdani, A.; Lekchiri, Y.; Morcellet., M.; Crini, G.; Morcellet, J.; Janus, L. New tetrapyrazolic macrocycle. Synthesis and preliminary use in metal ion extraction. Tetrahedron 2004, 60, 939–942, doi:10.1016/j.tet.2003.11.029.
- Xie, F.; Lin, X.; Wu, W.; Xie, Z. Solid phase extraction of lead (II), copper (II), cadmium (II), and nickel (II) using gallic acid modified silica gel prior to determination by flame atomic adsorption spectrometry. Talanta 2008, 74, 836–843, doi:10.1016/j.talanta.2007.07.018.
- Chiron, N.; Guilet, R. Adsorption of Cu(II) and Pb(II) onto a grafted silica: Isotherms and kinrtic models. Water Res. 2003, 37, 3079–3086, doi:10.1016/S0043-1354(03)00156-8.
- Radi, S.; Ramdani, A.; Lekchiri, Y.; Morcellet, M.; Crini, G.; Janus, L.; Bacquet, M. Immobilization of pyrazole compounds on silica gels and their preliminary use in metal ion extraction. New J. Chem. 2003, 27, 1224–1227, doi:10.1039/b301111a.
- Radi, S.; Attayibat, A. Functionalized SiO2 with S-donor thiophene. Synthesis, Characterisation and its heavy metals adsorption. Phos. Sul. Sil. Relat. Elem. 2010, 185, 2003–2013.
- Kocjan, R. Retention of some metal ions and their separation on silica gel modified with Acid Red 88. Mikrochim. Acta 1999, 131, 153–158, doi:10.1007/s006040050021.
- Zaporozhets, O.; Petruniock, N.; Sukhan, V. Determination of Ag(I), Hg(II) and Pb(II) by using silica gel loeded with dithizone and zinc dithizonate. Talanta 1999, 50, 865–873, doi:10.1016/S0039-9140(99)00172-1.
- Huang, X.; Chang, X.; He, Q.; Cui, Y.; Zhai, Y.; Jiang, N. Tris(2-aminoethyl) amine functionalized silica gel for solid-phase extraction and preconcentration of Cr(II), Cd(II), and Pb(II) from waters. J. Haz. Mater. 2008, 157, 154–160, doi:10.1016/j.jhazmat.2007.12.113.
- Shahbazi, A.; Younesi, H.; Badiei, A. Functionalized SBA-15 mesoporous silica by melamine-based dendrimer amines for adsorptive characteristics of Pb(II), Cu(II) and Cd(II) heavy metal ions in batch and fixed bed column. Chem. Eng. J. 2011, 168, 505–518, doi:10.1016/j.cej.2010.11.053.
- Fifani, J.; Ramdani, A.; Tarrago, G. 1,6,11,16-Tetraazaporphyrinogen, Synthesis and behavior. New J. Chem. 1977, 1, 521–528.
- Tarrago, G.; Ramdani, A.; Elguero, J.; Espada, M. Orientation de la réaction d'alkylation des pyrazoles dans des conditions neutres et en catalyse par transfert de phase. J. Heterocycl. Chem. 1980, 17, 137–142, doi:10.1002/jhet.5570170128.
- Sample Availability: Samples of the adsorbents are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).