- Research article
- Open Access
Synthesis and application of Amberlite xad-4 functionalized with alizarin red-s for preconcentration and adsorption of rhodium (III)
© Sid Kalal et al.; licensee BioMed Central Ltd. 2012
- Received: 7 June 2012
- Accepted: 20 July 2012
- Published: 18 September 2012
A new chelating resin was prepared by coupling Amberlite XAD-4 with alizarin red-s through an azo spacer, characterized by infra-red spectroscopy and thermal analysis and studied for Rh(III) preconcentration using inductively coupled plasma atomic emission spectroscopy (ICP-AES) for rhodium monitoring in the environment. The optimum pH for sorption of the metal ion was 6.5. The sorption capacity was found 2.1 mg/g of resin for Rh(III). A recovery of 88% was obtained for the metal ion with 1.5 M HCl as eluting agent. Kinetic adsorption data were analyzed by adsorption and desorption times of Rh(III) on modified resin. Scat chard analysis revealed that the homogeneous binding sites were formed in the polymers. The linear regression equation was Q/C = –1.3169Q + 27.222 (R2 = 0.9239), for Rh were formed in the SPE sorbent,Kd and Qmax for the affinity binding sites were calculated to be 0.76 μmol/mL and 20.67 μmol/g, respectively. The equilibrium data and parameters of Rh(III) adsorption on modified resin were analyzed by Langmuir, Freundlich, Temkin and Redlich–Peterson models. The experimental adsorption isotherm was in good concordance with Langmuir and Freundlich models (R2 > 0.998) and based on the Langmuir isotherm the maximum amount of adsorption (qmax) was 4.842 mg/g. The method was applied for rhodium ions determination in environmental samples. with high recovery (>80%).
- Environmental measurement
- Solid phase extraction
- Amberlite XAD-4
- Rhodium, Immobilization
The interest in ligand immobilized solid phase like silica gel [1, 2], organic polymer or copolymers, cellulose [3, 4] and polyurethane foam  continues because of their several applications, for example in solid phase metal extraction , designing hybrid organic–inorganic catalysts  and heterogenization of homogeneous catalysts . Solid phase extraction of metal ions present at trace level in environmental samples, high purity materials, biological samples and other complex matrices, makes the analytical techniques possible, such as flame atomic absorption spectrometry (FAAS) and inductive couple plasma atomic emission spectroscopy (ICP-AES). Solid phase extraction is preferable over ion exchange and solvent extraction due to its advantages like selectivity by controlling pH, reusability, high pre concentration factors, durability, versatility and metal loading capacity [9–13].
Adsorption of metal ions is widely used in the removal of contaminants from wastewaters. The design and efficient operation of adsorption processes require equilibrium adsorption data. The equilibrium isotherm plays an important role in predictive modeling for analysis and design of adsorption systems.
Amberlite XAD resin are widely used for modification with chelating materials due to its good physical and chemical properties such as porosity, high surface area, durability and purity. Many ligands, such as chronotropic acid , α-nitrozo β-naphtol , salicylic acid , pyrocathecol , 1-(2-pyridiazo)-2-naphtol , O-amino benzoic acid , 2-(methylthio) aniline , 3,4-dihydroxybenzoic acid , 2-aminothiophenol , and succinic acid  were covalently coupled with a polymer backbone through an azo (-N = N-) [24, 25], methylene (-CH2-)  or other groups [27, 28]. There are many reports of functionalized Amberlite XAD 2, 4 and 7 resins in this respect [29–37].
rhodium (Rh) is present at about 0.001 mg/L in the earth’s crust. Metallic rhodium metal is known for its stability in corrosive environments, physical beauty and unique chemical properties. It commands a premium price because of its low abundance in nature. Rhodium is now widely used in combination with platinum. Rhodium is commonly used for alloying platinum in thermocouples, crucibles, evaporating dishes, weighing boats windings for high-temperature furnaces. It finds applications as a coating material because of the hardness and luster of its surface. Because of its commercial importance, a wide variety of reagents have been proposed for preconcentration of Rh before spectrophotometric determination.
In this work, Amberlite XAD-4 alizarin red-s was prepared by chemically bonding to be used as an adsorbent. Alizarin red-s could form chelates with metallic ions on the surface of the resin. Adsorption of Rh(III) from aqueous solution and isotherm study using modified Amberlite XAD-4 was investigated under different experimental conditions to assess its affinity towards the chelator.
pH measurements were made with Metrohm model 744 (Switzerland) pH meter. IR spectra were recorded on a FT-IR spectrometer (Jasco/FT-IR-410) by KBr pellet method. Elemental analysis was carried out on an elemental analyzer from Thermo-Finnegan (Milan, Italy) model Flash EA. ICP-AES Varian, Vista-pro (Salt lake city, USA) was used for measuring the concentration of Rh (III). Thermo gravimetric analysis (TGA) was carried out by using TGA-50 H (Shimadzu, Japan).
Reagents and solutions
Acetic acid, sodium acetate, sodium hydrogen phosphate, sodium dihydrogen phosphate, rhodium chloride, tin (II) chloride, hydrochloric acid, sulfuric acid, nitric acid, sodium nitrite, sodium hydroxide, alizarin red, and iodide-starch paper were products of Merck Co. (Darmstadt, Germany).
All of the solutions were prepared in demonized water using analytical grade reagents. The stock solution (500 mg/L) of Rh(III) was prepared by dissolving appropriate amounts of rhodium chloride respectively in demonized water. 10 mL 0.01 M acetic acid acetate buffer (pH = 3-5) and 0.01 M phosphate buffer (pH = 6-9) were used to adjust pH of the solutions, wherever suitable. Amberlite XAD-4 resin (surface area = 745 m2/g, pore diameter = 5 nm and bead size = 20-60 mesh) was obtained from Serve (Heidelberg, New York).
Synthesis of chelating resin
Amberlite XAD-4 beads (5 g) were treated with 10 mL of concentrated HNO3 and 25 mL of concentrated H2SO4 and the mixture was stirred at 60°C for 1 hour on an oil bath. Then, the reaction mixture was poured into an ice water mixture. The nitrated resin was filtered, washed repeatedly with water until free from acid and then treated with a reducing mixture of 40 g of SnCl2, 45 mL of concentrated HCl and 50 mL of ethanol. The mixture was refluxed for 12 hours at 90°C. The solid precipitate was filtered and washed with water and 2 mol/L NaOH which released amino resin (R-NH2) from (RNH3)2 SnCl6 (R = resin matrix). The amino resin was first washed with 2 mol/L HCl and finally with distilled water to remove the excess HCl. It was suspended in an ice-water mixture (350 mL) and treated with 1 mol/L HCl and 1 mol/L NaNO2 (added in small aliquots of 1 mL) until the reaction mixture showed a permanent dark blue color with starch-iodide paper. The diazotized resin was filtered, washed with ice-cold water and reacted with alizarin red-s 0.03 mol in 30 mL 2 mol/L HCl, respectively. The reaction mixture was stirred at 0-3°C for 24 hours. Then, the resulting colored beads were filtered, washed with water and dried in air.
A sample solution (50 mL) containing (0.3 μg/mL) of Rh (III) was taken in a glass stopped bottle, after adjusting its pH to the optimum value. The 0.05 g of alizarin red S-Amberlite XAD-4 was added to the bottle and the mixture was shaken for optimum time. The resin was filtered and sobbed metal ion was eluted with 1.5 M HCl (10 mL). The concentration of metal ion in the elute solution was determined by ICP-AES. The wavelength of 343 nm was used for Rh determination.
Isotherm studies of Rh (III) adsorption
Where C0 and Ce(mg/L) are initial and equilibrium concentrations of Rh(III), respectively; V (L) is the volume of the solution and m (g) is the mass of the adsorbent used.
Metal sorption as a function of pH
The degree of metal sorption at different pH values was determined by batch equilibration technique. A set of solutions (volume of each = 100 mL) containing 0.3 μg/mL of Rh(III) was taken. pH was adjusted in the range of 3-9 with 0.01 M acetate and/or phosphate buffer solutions. 0.1 g of alizarin red S-Amberlite XAD-4 was added to each solution and the mixture was shaken for 4 hours. The optimum pH for quantitative uptake of metal ions was ascertained by measuring Rh (III) content (by ICP-AES) in supernatant liquid and in the elute obtained by desorbing the metal ion from resin with 1.5 M hydrochloric acid (10 mL).
0.05 g of poly(AGE/IDA-co-DMAA)-grafted silica gel was stirred for 4 h. with 50 mL solution containing10-50 μg/mL of Rh(III) at optimum pH and 20°C. The metal ion concentration in the supernatant liquid was estimated by ICP-AES. The sorption capacity of the sorbent for the metal ion was ascertained from the difference between the metal ion concentration in the solution before and after the sorption.
Optimization of sorption time of rhodium ions
Alizarin red-s-amberliteXAD-4 (0.1 g) was shaken with 50 mL of solution containing 300 μg/mL of Rh (III) for different times (20, 60, 90, 120, 150 and 180 min) under optimum pH. After taking out the sorbent, concentration of rhodium ions in the solution was determined with ICP-AES using recommended batch method.
Isotherm parameters obtained by using non-linear method
Langmuir isotherm model
Freundlich isotherm model
Temkin isotherm model
Redlich–Peterson isotherm model
Table 1 shows that the value of RL (0.5764)is in the range of 0-1 at optimum pH which confirms the favorable uptake of the Rh(III).
Therefore, a plot of ln(qe) versus ln(Ce) (Figure 1B) enables the constant KF and exponent 1/n to be determined. The Freundlich equation predicts that the Rh(III) concentration on the adsorbent will increase as long as there is an increase in Rh(III) concentration in the liquid.
The Temkin equation suggests a linear decrease of sorption energy as the degree of completion of the optional centers of an adsorbent is increased.
Where B = RT/b and b is the Temkin constant related to heat of sorption (J/mol). A is the Temkin isotherm constant (L/g), R is the gas constant (8.314 J/mol K) and T is the absolute temperature (K). Therefore plotting qe versus ln(Ce) (Figure 1C) enables one to determine the constants A and B. Temkin parameters calculated from Equations (7 and 8) are listed in Table 1.
It has three isotherm constants, namely, A, B, and g (0 < g < 1), which characterize the isotherm. The limiting behavior can be summarized as follows:
i.e. the Langmuir form results.
i.e. the Freundlich form results.
i.e. the Henry’s Law form results.
Three isotherm constants, A, B, and g can be evaluated from the linear plot represented by Eq. (13) using a trial and error procedure. It was developed to determine the isotherm parameters by optimization routine to maximize the coefficient of determination, R2, for a series of values of A for the linear regression of ln(Ce) on ln[A(Ce/qe) − 1] and to obtain the best value of A which yields a maximum ‘optimized’ value of R 2 using the solver add-in with Microsoft’s spreadsheet, Microsoft Excel(Figure 1D).
Scat chard analysis was employed to further analyze the binding isotherms, which is an approximate model commonly used in SPE (Solid Phase Extraction) characterization. The Scat chard equation can be expressed as, Q/C = (Qmax–Q)/Kd, where C (μmol/mL) is the equilibrium concentration of rhodium; Q (μmol/g) is the equilibrium adsorption amount at each concentration Qmax (μmol/g) is the maximum adsorption amount; and Kd (μmol/mL) is the equilibrium dissociation constant at binding sites.
TGA of the resins shows two step weight losses up to 510°C. The weight loss up to 130°C was due to the water molecules in the polymer. The major weight loss after 290°C is due to the dissociation of chemically immobilized moiety and the polymeric matrix.
Stability and reusability of the adsorbent resin
Rh(III) was absorbed and desorbed on 1 g of the resin several times. It was found that sorption capacity of resin after 10 cycles of its equilibration with Rh(III), changes less than 5%. Therefore, repeated use of the resin is feasible. The resin cartridge after loading it with samples can be readily regenerated with 1.5 M HCl. The sorption capacity of the resin stored for more than 6 months under ambient conditions has been found to be practically unchanged.
Optimization of desorption time of rhodium ions
The Redlich–Peterson isotherm constants, A, B, and g as well as the coefficient of determination, R2, for the sorption of Rh(III) onto Alizarin red S-Amberlite XAD-4 using the linear regression is shown in Table 1. It can be seen that the values of (g) were close to unity, which means that the isotherms are approaching the Langmuir form and not the Freundlich isotherm. The result shows that the Langmuir isotherm best-fit the equilibrium data for adsorption of Rh(III) on Alizarin red S-Amberlite XAD-4.
Scat chard analysis
Application of method
Results obtained for Rh(III)measurement in tap water (I) and spring water (II)
Found (without spiking of Rh(III))
Added Rh(III) (μg/mL)
Found Rh(III), after preconcentration (μg/mL)
Relative standard deviation (%) a
The experimental FT-IR spectrum and thermal analysis (TGA) show this resin was satisfactory for adsorption of rhodium ion. In pH = 6.5, the results indicated 88% recovery and at 15 min (Figure 5) the adsorption of resin was about 50%. The equilibrium data and parameters of Rh(III) adsorption on modified resin were analyzed by Langmuir, Freundlich, Temkin and Redlich–Peterson models. The experimental adsorption isotherm was in good concordance with Langmuir and Freundlich models (R2 > 0.998) and based on the Langmuir isotherm the maximum amount of adsorption (qmax) was 4.842 mg/g.
The results about application of this work in real sample and environmental studies, also demonstrated the applicability of the procedure for rhodium determination in real samples with high recovery (>80%).
The authors gratefully acknowledge the support of this work by Faculty of Sciences, Tarbiat Modares University.
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