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.
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.
- Anderson JL, Armstrong DW, Wei G: Ionic liquids in analytical chemistry. Anal Chem 2006, 78: 2893–2902.Google Scholar
- Awwad NS, Gad HM, Ahmad MI, Aly HF: Sorption of lanthanum and erbium from aqueous solution by activated carbon prepared from rice husk. Colloids Surf B 2010, 81: 593–599.View ArticleGoogle Scholar
- Ayata S, Bozkurt SS, Ocakoglu K: Separation and preconcentration of Pb(II) using ionic liquid-modified silica and its determination by flame atomic absorption spectrometry. Talanta 2011, 84: 212–215.View ArticleGoogle Scholar
- Aydin FA, Soylak M: A novel multi-element coprecipitation technique for separation and enrichment of metal ions in environmental samples. Talanta 2007, 73: 134–141.View ArticleGoogle Scholar
- Cai Y, Jiang G, Liu J, Zhou Q: Multiwalled carbon nanotubes as a solid-phase extraction adsorbent for the determination of bisphenol A, 4-n-nonylphenol, and 4-tert-octylphenol. Anal Chem 2003, 75: 2517–2521.View ArticleGoogle Scholar
- Chen Q: Study on the adsorption of lanthanum (III) from aqueous solution by bamboo charcoal. J Rare Earths 2010, 28: 125–131.View ArticleGoogle Scholar
- Cui Y, Chang X, Zhu X, Luo H, Hu Z, Zou X, He Q: Chemically modified silica gel with p-dimethylaminobenzaldehyde for selective solid-phase extraction and preconcentration of Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) by ICP-OES. Microchem J 2007, 87: 20–26.View ArticleGoogle Scholar
- Dean JR: Extraction methods for environmental analysis. New York: Wiley; 1998.Google Scholar
- Duran C, Gundogdu A, Bulut VN, Soylak M, Elci L, Sentürk HB, Tüfekci M: Solid-phase extraction of Mn(II), Co(II), Ni(II), Cu(II), Cd(II) and Pb(II) ions from environmental samples by flame atomic absorption spectrometry (FAAS). J Hazard Mater 2007, 146: 347–355.View ArticleGoogle Scholar
- Farajzadeh MA, Bahram M, Zorita S, Mehr BG: Optimization and application of homogeneous liquid-liquid extraction in preconcentration of copper (II) in a ternary solvent system. J Hazard Mater 2009, 161: 1535–1543.View ArticleGoogle Scholar
- Freundlich H: Über die adsorption in lösungen (adsorption in solution). Z Phys Chem 1906, 57: 384–470.Google Scholar
- Gaikwad AG, Damodaran AD: Solvent extraction studies of holmium with acidic extractants. Sep Sci Technol 1993, 28: 1019–1030.View ArticleGoogle Scholar
- Han DM, Fang GZ, Yan XP: 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, 1100: 131–136.View ArticleGoogle Scholar
- Handy ST: Room temperature ionic liquids: different classes and physical properties. Curr Org Chem 2005, 9: 959–988.View ArticleGoogle Scholar
- Jain VK, Handa A, Sait SS, Shrivastav P, Agrawal YK: Pre-concentration, separation and trace determination of lanthanum(III), cerium(III), thorium(IV) and uranium(VI) on polymer supported o-vanillinsemicarbazone. Anal Chim Acta 2001, 429: 237–246.View ArticleGoogle Scholar
- Jal PK, Patel S, Mishra BK: Chemical modification of silica surface by immobilization of functional groups for extractive concentration of metal ions. Talanta 2004, 62: 1005–1028.View ArticleGoogle Scholar
- Khajeh M: Response surface modelling of lead pre-concentration from food samples by miniaturised homogenous liquid-liquid solvent extraction: Box-behnken design. Food Chem 2011, 129: 1832–1838.View ArticleGoogle Scholar
- Koel M: Ionic liquids in chemical analysis. Boca Raton: CRC Press; 2009.Google Scholar
- Komjarova I, Blust R: Comparison of liquid-liquid extraction, solid-phase extraction and co-precipitation preconcentration methods for the determination of cadmium, copper, nickel, lead and zinc in seawater. Anal Chim Acta 2006, 576: 221–228.View ArticleGoogle Scholar
- Langmuir I: The constitution and fundamental properties of solids and liquids. J Am Chem Soc 1916, 38: 2221–2295.View ArticleGoogle Scholar
- Liang P, Peng L: Ionic liquid-modified silica as sorbent for preconcentration of cadmium prior to its determination by flame atomic absorption spectrometry in water samples. Talanta 2010, 81: 673–677.View ArticleGoogle Scholar
- Maanan M: Heavy metal concentrations in marine molluscs from the Moroccan coastal region. Environ Pollut 2008, 153: 176–183.View ArticleGoogle Scholar
- Maestro P, Huguenin D: Industrial applications of rare earths: which way for the end of the century. J Alloys Compd 1995, 225: 520–528.View ArticleGoogle Scholar
- Mahmoud ME: Surface loaded 1-methyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM + Tf 2 N - ] hydrophobic ionic liquid on nano-silica sorbents for removal of lead from water samples. Desalination 2011, 266: 119–127.View ArticleGoogle Scholar
- Mahmoud ME, Al-Bishri HM: Supported hydrophobic ionic liquid on nano-silica for adsorption of lead. Chem Eng J 2011, 166: 157–167.View ArticleGoogle Scholar
- Manzoori JL, Abdolmohammad-Zadeh H, Amjadi M: Ultra-trace determination of silver in water samples by electrothermal atomic absorption spectrometry after preconcentration with a ligand-less cloud point extraction methodology. J Hazard Mater 2007, 144: 458–463.View ArticleGoogle Scholar
- Marahel F, Ghaedi M, Montazerozohori M, Nejati Biyareh M, Nasiri Kokhdan S, Soylak M: Solid-phase extraction and determination of trace amount of some metal ions on duolite XAD 761 modified with a new Schiff base as chelating agent in some food samples. Food Chem Toxicol 2011, 49: 208–214.View ArticleGoogle Scholar
- Marwani HM: Spectroscopic evaluation of chiral and achiral fluorescent ionic liquids. Cent Eur J Chem 2010, 8: 946–952.View ArticleGoogle Scholar
- Marwani HM: Exploring spectroscopic and physicochemical properties of new fluorescent ionic liquids. J Fluoresc 2013, 23: 251–257.View ArticleGoogle Scholar
- McKay G, Blair HS, Gardner JR: Adsorption of dyes on chitin. I. Equilibrium studies. J Appl Polym Sci 1982, 27: 3043–3057.View ArticleGoogle Scholar
- Ngeontae W, Aeungmaitrepirom W, Tuntulani T, Imyim A: Highly selective preconcentration of Cu(II) from seawater and water samples using amidoamidoxime silica. Talanta 2009, 78: 1004–1010.View ArticleGoogle Scholar
- Palmieri MC, Volesky B, Garcia O Jr: Biosorption of lanthanum using sargassum fluitans in batch system. Hydrometallurgy 2002, 67: 31–36.View ArticleGoogle Scholar
- Qiao R, Zhang R, Zhu W, Gong P: Lab simulation of profile modification and enhanced oil recovery with a quaternary ammonium cationic polymer. J Ind Eng Chem 2012, 18: 111–115.View ArticleGoogle Scholar
- Quang DV, Kim JK, Sarawade PB, Tuan DH, Kim HT: Preparation of amino-functionalized silica for copper removal from an aqueous solution. J Ind Eng Chem 2012, 18: 83–87.View ArticleGoogle Scholar
- Rao MM, Kumar Reddy DHK, Venkateswarlu P, Seshaiah K: Removal of mercury from aqueous solutions using activated carbon prepared from agricultural by-product/waste. J Environ Manage 2009, 90: 634–643.View ArticleGoogle Scholar
- Safavi A, Abdollahi H, Hormozi Nezhad MR, Kamali R: Cloud point extraction, preconcentration and simultaneous spectrophotometric determination of nickel and cobalt in water samples. Spectrochim Acta Part A 2004, 60: 2897–2901.View ArticleGoogle Scholar
- Saracoglu S, Soylak M, Kacar Peker DS, Elci L, dos Santos WNL, Lemos VA, Ferreira SLC: A pre-concentration procedure using coprecipitation for determination of lead and iron in several samples using flame atomic absorption spectrometry. Anal Chim Acta 2006, 575: 133–137.View ArticleGoogle Scholar
- Sert S, Kütahyali C, Inan S, Talip Z, Cetinkaya B, Eral M: Biosorption of lanthanum and cerium from aqueous solutions by platanus orientalis leaf powder. Hydrometallurgy 2008, 90: 13–18.View ArticleGoogle Scholar
- Shin EM, Choi HS: Column preconcentration and determination of cobalt(II) using silica gel loaded with 1-nitroso-2-naphthol. Bulletin of the Korean Chemical Society 2009, 30: 1516–1520.View ArticleGoogle Scholar
- Simpson NJK: Solid phase extraction: principles, strategies and applications. New York: Marcel Dekker; 2000.View ArticleGoogle Scholar
- Soylak M, Erdogan ND: Copper(II)-rubeanic acid coprecipitation system for separation-preconcentration of trace metal ions in environmental samples for their flame atomic absorption spectrometric determinations. J Hazard Mater 2006, 137: 1035–1041.View ArticleGoogle Scholar
- Tao GH, Fang Z: Dual stage preconcentration system for flame atomic absorption spectrometry using flow injection on-line ion-exchange followed by solvent extraction. J Anal Chem 1998, 360: 156–160.View ArticleGoogle Scholar
- Tong S, Zhao S, Zhou W, Li R, Jia Q: Modification of multi-walled carbon nanotubes with tannic acid for the adsorption of La, Tb and Lu ions. Microchim Acta 2011, 174: 257–264.View ArticleGoogle Scholar
- Unlü N, Ersoz M: Adsorption characteristics of heavy metal ions onto a low cost biopolymeric sorbent from aqueous solutions. J Hazard Mater 2006, 136: 272–280.View ArticleGoogle Scholar
- Vidal L, Riekkola M-L, Canals A: Ionic liquid-modified materials for solid-phase extraction and separation: a review. Anal Chim Acta 2012, 715: 19–41.View ArticleGoogle Scholar
- Vijayaraghavan K, Sathishkumar M, Balasubramanian R: Biosorption of lanthanum, cerium, europium, and ytterbium by a brown marine alga, turbinaria conoides. Ind Eng Chem Res 2010, 49: 4405–4411.View ArticleGoogle Scholar
- Wang Z, Zhang Q, Kuehner D, Xu X, Ivaska A, Niua L: The synthesis of ionic-liquid-functionalized multiwalled carbon nanotubes decorated with highly dispersed Au nanoparticles and their use in oxygen reduction by electrocatalysis. Carbon 2008, 46: 1687–1692.View ArticleGoogle Scholar
- Wu F-C, Tseng R-L, Juang R-S: Kinetic modeling of liquid-phase adsorption of reactive dyes and metal ions on chitosan. Water Res 2001, 35: 613–618.View ArticleGoogle Scholar
- Zang Z, Li Z, Zhang L, Li R, Hu Z, Chang X, Cui Y: Chemically modified attapulgite with asparagine for selective solid-phase extraction and preconcentration of Fe(III) from environmental samples. Anal Chim Acta 2010, 663: 213–217.View ArticleGoogle Scholar
- Zhang L, Ding S-D, Yang T, Zheng G-C: Adsorption behavior of rare earth elements using polyethyleneglycol (phosphomolybdate and tungstate) heteropolyacid sorbents in nitric solution. Hydrometallurgy 2009, 99: 109–114.View ArticleGoogle Scholar
- Zhang N, Peng H, Hu B: Light-induced pH change and its application to solid phase extraction of trace heavy metals by high-magnetization Fe 3 O 4 @SiO 2 @TiO 2 nanoparticles followed by inductively coupled plasma mass spectrometry detection. Talanta 2012, 94: 278–283.View ArticleGoogle Scholar
- Zhao H: Innovative applications of ionic liquids as "green" engineering liquids. Chem Eng Commun 2006, 193: 1660–1677.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.