New solid phase extractor based on ionic liquid functionalized silica gel surface for selective separation and determination of lanthanum
© Marwani and Alsafrani; licensee Springer. 2013
Received: 20 April 2013
Accepted: 2 September 2013
Published: 11 September 2013
Direct determination of metal ions, in particular at ultra-trace concentration, cannot be easily achieved in complex systems by analytical techniques because of the lack of sensitivity and selectivity of these methods. Therefore, an efficient separation step is often required prior to the determination of metal ions for sensitive, accurate, and interference-free determination of metal ions. In accordance, a new solid phase extractor based on silica gel functionalized with ionic liquid (SG-N-PhenacylPyrNTf2) was developed for a selective separation of La(III) prior to its determination by inductively coupled plasma-optical emission spectrometry.
Immobilization of the ionic liquid on activated silica gel surface was confirmed by both Fourier transform infrared spectroscopy and scanning electron microscope. The concentration of ionic liquid on the surface of activated silica gel was determined based on thermal desorption method. The uptake behavior of the new SG-N-PhenacylPyrNTf2 adsorbent toward metal ions was studied under static conditions by batch mode. The supernatant concentrations of metal ions were directly determined after filtration by inductively coupled plasma-optical emission spectrometry.
Fourier transform infrared spectroscopy and scanning electron microscopy results strongly confirmed the formation of SG-N-PhenacylPyrNTf2 phase. Adsorption isotherm study revealed the preference of SG-N-PhenacylPyrNTf2 over activated silica gel for a selective separation of La(III) prior to its determination by inductively coupled plasma-optical emission spectrometry. Adsorption isotherm data were well fit the Langmuir adsorption model with a maximum adsorption capacity of 165.39 mg g−1 for La(III), which was consistent with that (167.08 mg g−1) experimentally obtained from adsorption isotherm study. Kinetic study demonstrated that the adsorption of La(III) on the SG-N-PhenacylPyrNTf2 phase followed the pseudo second-order kinetic model.
Ultimately, the developed method can be applied and effectively utilized for the determination of La(III) in natural water samples with acceptable and reliable results.
KeywordsN-PhenacylPyrNTf2; Silica gel surface; La(III) Adsorption; ICP-OES; Batch mode
Rare earth metals (REMs) have been used in different fields, such as chemical engineering, metallurgy, nuclear energy, optical, magnetic, luminescence, and laser materials, high-temperature superconductors, secondary batteries, and catalysis (Maestro and Huguenin 1995; Gaikwad and Damodaran 1993). Lanthanum is one of the REMs and has received special interest due to its technological importance and increasing demands for advanced new materials. Present day applications of lanthanum, as a pure element or in association with other compounds, are in super alloys, catalysts, special ceramics, and organic synthesis (Palmieri et al. 2002). However, most of the environmental problems are usually caused by human activities and natural resources. In recent years, this phenomenon is remarkably extended with the population density and growth of technology. There are several contaminants in nature, such as contamination with heavy metals, radionuclides, and lanthanides (Maanan 2008; Awwad et al. 2010). All of these are toxic and harmful to the public health, even at low concentration level (Cui et al. 2007). Thus, the most convenient way to overcome these problems is to apply selective extraction techniques, particularly when they exist at ultra-trace concentration level in complex matrix (Zang et al. 2010).
Various extraction techniques were implemented for the quantitative determination of trace metal ions, including liquid-liquid extraction (Khajeh 2011; Farajzadeh et al. 2009), coprecipitation (Komjarova and Blust 2006; Soylak and Erdogan 2006; Saracoglu et al. 2006), ion exchange (Tao and Fang 1998), cloud point extraction (Manzoori et al. 2007; Safavi et al. 2004), and solid-phase extraction (Zhang et al. 2012; Marahel et al. 2011; Duran et al. 2007). Recently, solid-phase extraction (SPE) has been widely used as a separation tool for the speciation of metal ions in environmental samples and has received much attention because of its advantages, such as absence of emulsion, high enrichment factor, disposal cost due to low consumption of reagent, and more importantly environment-friendly (Aydin and Soylak 2007; Simpson 2000). Nevertheless, the main limitation of SPE is the lack of selectivity (Jal et al. 2004), which leads to high interference of the other existing species with the target metal ion. The choice of a proper adsorbent plays an important role in SPE because it can control the analytical parameters, such as selectivity, affinity, and capacity (Cai et al. 2003; Dean 1998). Hence, different surface modification methods have been applied to classical SPE adsorbents (such as silica (Shin and Choi 2009) and polymer (Qiao et al. 2012)) in order to increase the selectivity.
Room-temperature ionic liquids (RTILs) have several unique chemical and physical properties which make it useful for wide applications in organic chemistry, inorganic chemistry, electrochemistry, and analytical chemistry (Zhao 2006; Vidal et al. 2012). RTILs have good thermal stability, negligible vapor pressure, tunable viscosity and miscibility with water, high conductivity, and high heat capacity (Handy 2005; Koel 2009). In addition, extraction and separation techniques applying solid adsorbents modified with ionic liquids (ILs) have become very active fields in analytical chemistry (Anderson et al. 2006). ILs can be immobilized on the surface of solid supports for additional applications as solid phase extractors of metal ions from their matrices or aqueous solutions (Mahmoud and Al-Bishri 2011). They have been immobilized on multiwalled carbon nanotubes (Wang et al. 2008), nanosilica (Mahmoud 2011; Mahmoud and Al-Bishri 2011), and silica gel (Ayata et al. 2011; Liang and Peng 2010). Silica gel (SG) adsorbents, as an example of inorganic solid phases, afford several advantages over organic solid phases, such as high porosity and hydrophilicity and ease of surface modification (Ngeontae et al. 2009; Quang et al. 2012). However, research studies were little in the abovementioned field of the applications of supported ionic liquid phases.
In accordance, a solid phase extractant (SG-N-PhenacylPyrNTf2) was developed based on a hybrid combination of the hydrophobic character of newly synthesized ionic liquid with SG properties, without the need for partition treatment by chelating compounds. Both Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscope (SEM) confirmed the formation of the resulted SG-N-PhenacylPyrNTf2 adsorbent. The selectivity of SG-N-PhenacylPyrNTf2 toward different metal ions, including Co(II), Fe(II), Fe(III), La(III), and Ni(II) was investigated. The uptake behavior of the new SG-N-PhenacylPyrNTf2 adsorbent toward La(III) was evaluated under batch conditions. Adsorption isotherm data were well fit with the Langmuir adsorption model, strongly supporting that the adsorption process was mainly monolayer on a homogeneous adsorbent surface. Kinetic study also demonstrated that the adsorption of La(III) on the SG-N-PhenacylPyrNTf2 phase obeyed the pseudo second-order kinetic model. The proposed method was ultimately applied to real water samples with satisfactory results.
Chemicals and reagents
All reagents used were of high purity and of analytical reagent grade, and doubly distilled deionized water was used throughout experiments. N-phenacylpyridinium bromide (N-PhenacylPyrBr), bis(trifluoromethane)sulfonimide lithium (LiNTf2), ethyl alcohol (Et-OH), and diethyl ether were purchased from Sigma-Aldrich (Milwaukee, WI, USA). SG (SiO2, particle size 10 to 20 nm) with purity of 99.5% was also obtained from Sigma-Aldrich. Lanthanum nitrate [La(NO3)3] and stock standard solutions of 1,000 mg L−1 of Co(II), Fe(II), Fe(III), and Ni(II) were purchased from Sigma-Aldrich.
Preparation of the new solid phase extractor
Preparation of N-PhenacylPyrNTf2 ionic liquid
N-PhenacylPyrNTf2 ionic liquid was prepared according to previously reported standard method by Marwani (Marwani 2013; 2010). Specifically, an amount of 2 g N-PhenacylPyrBr was separately weighed and dissolved in 18.2 MΩ·cm distilled deionized water. The N-PhenacylPyrBr solution was then mixed with an equimolar amount of LiNTf2. The resultant reaction mixture was stirred for 2 h at room temperature, and the reaction resulted in two phases. The lower layer was separated and dried under vacuum overnight.
Activation of SG
SG powder (25.0 g) was first activated by refluxing and stirring with 200 mL of 50% (v/v) concentrated hydrochloric acid solution for 8 h in order to remove metal oxide and nitrogenous impurity and to enhance the content of silanol groups on the SG surface. The activated SG powder was filtered, repeatedly washed with 18.2 MΩ·cm distilled deionized water until acid-free and oven dried at 120°C for 5 h to remove surface-adsorbed water.
Synthesis of SG-N-PhenacylPyrNTf2
In this study, the effect of solution pH, adsorption capacity determination, effect of contact time, and effect of coexisting ions were investigated under static conditions by batch mode. For the effect of pH on the selectivity of SG-N-PhenacylPyrNTf2 toward selected metal ions, standard solutions of 2 mg L−1 of each metal ion were prepared and adjusted to pH values ranging from 1.0 to 9.0, except for Fe(II) and Fe(III), with a series of buffer solutions, 0.2 mol L−1 HCl/KCl for pH 1.0 and 2.0, 0.1 mol L−1 CH3COOH/CH3COONa for pH 3.0 to 6.0, and 0.1 mol L−1 Na2HPO4/HCl for pH 7.0 to 9.0. Both Fe(II) and Fe(III) were prepared at the same concentration as above but were only studied with buffer solutions at pH values ranging from 1.0 to 4.0 in order to avoid any precipitation with buffer solutions at pH 5.0 to 9.0. Each standard solution was individually mixed with 20 mg of dry SG-N-PhenacylPyrNTf2 phase. All mixtures were shaken vigorously at room temperature for 1 h to facilitate adsorption of the metal ions onto the SG-N-PhenacylPyrNTf2. The supernatant concentration of metal ions was determined directly by inductively coupled plasma-optical emission spectrometry (ICP-OES) after filtration. For the study of La(III) adsorption capacity, standard solutions of 0, 5, 10, 20, 30, 40, 60, 80, 100, 125, 150, 200, and 500 mg L−1 were prepared as above, adjusted to the optimum pH value of 6.0 with the buffered aqueous solution (0.1 mol L−1 CH3COOH/CH3COONa) and individually mixed with 20 mg SG-N-PhenacylPyrNTf2. The mixtures were mechanically shaken for 1 h at room temperature. In addition, the effect of shaking time on the La(III) uptake capacity was studied under the same batch conditions but at different equilibrium periods (2.5, 5, 10, 20, 30, 40, 50, and 60 min).
FT-IR spectra were recorded before and after modification of the SG phase on Perkin Elmer spectrum 100 series FT-IR spectrometer (Beaconsfield, Bucks, UK) in the range 4,000 to 600 cm−1. A Jenway model 3505 laboratory pH meter (CamLab, Cambridgeshire, UK) was employed for the pH measurements and was calibrated with standard buffer solutions. Surface morphologies of SG before and after modification were investigated by SEM on a field emission scanning electron microscope (QUANT FEG 450, Amsterdam, Netherlands). The microscope was operated at an accelerating voltage of 15 kV. Thermolyne 47900 furnace was used to determine the millimoles per gram surface coverage value of SG-N-PhenacylPyrNTf2 surface by thermal desorption analysis. ICP-OES measurements were acquired with the use of a Perkin Elmer ICP-OES model Optima 4100 DV, Shelton, CT, USA. The ICP-OES instrument was optimized daily before measurement and operated as recommended by the manufacturers. The ICP-OES spectrometer was used with the following parameters: FR power, 1,300 kW; frequency, 27.12 MHz; demountable quartz torch, Ar/Ar/Ar; plasma gas (Ar) flow, 15.0 L min−1; auxiliary gas (Ar) flow, 0.2 L min−1; nebulizer gas (Ar) flow, 0.8 L min−1; nebulizer pressure, 2.4 bar; glass spray chamber according to Scott (Ryton), sample pump flow rate, 1.5 mL min−1; integration time, 3 s; replicates, 3; and wavelength range of monochromator 165 to 460 nm. The wavelengths selected for the determination of Fe(II and III), Ni(II), Co(II), and La(III) were 238.204, 231.604, 228.616, and 348.902 nm, respectively.
Results and discussion
Determination of the surface coverage value of the SG-N-PhenacylPyrNTf2 phase
An amount of 100 mg SG-N-PhenacylPyrNTf2 adsorbent was weighed in a dry porcelain crucible. The weighed amount was then gradually heated into a furnace from 50°C to 700°C, and the ignited phase was kept at this temperature for 1 h. The remaining SG-N-PhenacylPyrNTf2 phase was left to cool inside the furnace and then transferred to a desiccator. The weight loss of hydrophobic ionic liquid was determined by the difference in sample masses before and after the process of thermal desorption. Based on thermal desorption method, the concentration of N-PhenacylPyrNTf2 was determined to be 0.38 mmol g−1 on the surface of activated SG.
FT-IR and SEM characterization of SG-N-PhenacylPyrNTf2
Effect of pH and selectivity study
Selectivity study of 20 mg SG-N-PhenacylPyrNTf 2 toward different metal ions
Concentration (mg L−1)
q e(mg g–1)
K d(mL g−1)
2.50 × 106
Determination of La(III) adsorption capacity
Comparison of SG-N-PhenacylPyrNTf 2 adsorption capacity for La(III) reported in the present study with other SPE materials
Adsorption capacity (mg g−1)
(Zhang et al. 2009)
(Zhang et al. 2009)
(Tong et al. 2011)
(Awwad et al. 2010)
Turbinaria conoides biomass
(Vijayaraghavan et al. 2010)
Powderized leaves of Platanus orientalis
(Sert et al. 2008)
(Jain et al. 2001)
Adsorption isotherm models
where K f and n are the Freundlich constants and can be calculated from the intercept and slope, respectively, of the linear plot of logq e versus logC e .
where C e corresponds to the equilibrium concentration of the metal ion in the supernatant (mg mL−1) and q e represents the amount of metal ion per gram of the adsorbent (mg g−1). The symbols Q o and b refer to Langmuir constants for SG-N-PhenacylPyrNTf2 and are related to the maximum La(III) adsorption capacity (mg g−1) and affinity parameter (L mg−1), respectively. Langmuir constants can be obtained from a linear plot of C e/q e against C e with a slope and intercept equal to 1/Q o and 1/Q o b, respectively. In addition, the essential characteristics of the Langmuir adsorption isotherm can be represented in terms of a dimensionless constant separation factor or equilibrium parameter, R L , which is defined as R L = 1/(1 + bC o), where b is the Langmuir constant, indicating the nature of adsorption and the shape of the isotherm, and C o is the initial concentration of the analyte of interest. The R L value indicates the type of the isotherm, and R L values lying between 0 and 1 indicates that the conditions were favorable for the adsorption process (McKay et al. 1982).
The corresponding fitting parameters of Q o and b of Langmuir isotherm model were also calculated and found to be 165.39 mg g−1 and 0.27 L mg−1, respectively. The R L value of La(III) adsorption on SG-N-PhenacylPyrNTf2 was also determined to be 0.02, supporting a highly favorable adsorption process based on the Langmuir adsorption isotherm model. It is also of interest to notice that the La(III) adsorption capacity (165.39 mg g−1) calculated from Langmuir equation was strongly consistent with that (167.08 mg g−1) experimentally obtained from the adsorption isotherm study.
Effect of contact time
where q e (mg g−1) and q t (mg g−1) are the amount of adsorption at equilibrium and at time t (min), respectively, and k 1 denotes the adsorption rate constant of pseudo first-order adsorption (min−1). The adsorption rate constant k 1 and adsorption capacity q e can be calculated from the slope and intercepts of the plot of log(q e – q t ) against t.
where υ o = k 2 is the initial adsorption rate (mg g–1 min–1) and k 2 (g mg–1 min–1) corresponds to the rate constant of the pseudo second-order adsorption; q e (mg g–1) is the amount of metal ion adsorbed at equilibrium, and q t (mg g–1) refers to the amount of metal ion on the adsorbent surface at any time t (min). Kinetic parameters of q e and υ o can be obtained from the slope and intercept, respectively, of the linear plots of t/q t versus t.
Performance of method in analytical applications
Effect of interfering ions
Effect of matrix interferences on the extraction of 1 mg L −1 La(III) on 20 mg SG-N-PhenacylPyrNTf 2 ( N = 3)
Concentration (mg L −1)
% Extraction of La(III)
Na+, K+, NH4 +
Cl−, F−, NO3 −
CO3 2−, SO4 2−
Application of the proposed method
Determination of La(III) at different concentrations in real water samples using 20 mg SG-N-PhenacylPyrNTf 2
Added (mg L−1)
Unadsorbed (mg L−1)
In this study, the immobilization of N-PhenacylPyrNTf2 on activated SG, as a new solid phase extractor (SG-N-PhenacylPyrNTf2), was successfully accomplished via electrostatic interaction. The SG-N-PhenacylPyrNTf2 phase attained a perfect selectivity for the extraction and determination of La(III) in aqueous solution even in the presence of plentiful interfering ions. Results also demonstrated that adsorption isotherm data for La(III) adsorption on the SG-N-PhenacylPyrNTf2 phase were well fit with the Langmuir classical adsorption isotherm model, providing that the formation of a monolayer over a homogeneous adsorbent surface. Moreover, kinetic isotherm data displayed that the adsorption of La(III) on the SG-N-PhenacylPyrNTf2 phase obeyed a pseudo second-order kinetic reaction. Ultimately, the developed method can be applied and effectively utilized for the determination of La(III) in natural water samples with acceptable and reliable results.
The authors gratefully acknowledge the Department of Chemistry and Center of Excellence for Advanced Materials Research (CEAMR) at King Abdulaziz University for providing research facilities.
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