Studies on Complexation and Supercritical Fluid Extraction of Cd2+

Mar 29, 2015 - In the present study, we explored the extraction of Cd2+ by supercritical fluid carbon dioxide using calixarenes as complexing agents i...
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Studies on Complexation and Supercritical Fluid Extraction of Cd2+ with Calixarenes Nilesh V. Rathod,† Ankita Rao,‡ Pradeep Kumar,‡ Karanam L. Ramakumar,‡ and Dipalee D. Malkhede*,† †

Department of Chemistry, Savitribai Phule Pune University, Pune 411007, India Radio Analytical Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai 400085, India

‡

S Supporting Information *

ABSTRACT: In the present study, we explored the extraction of Cd2+ by supercritical fluid carbon dioxide using calixarenes as complexing agents in acidic medium. The optimum conditions for the extraction of cadmium were found to be 30 min of static extraction followed by 30 min of dynamic extraction, a temperature of 323 K, a pressure of 20.27 MPa, a CO2 flow rate of 2 mL min−1, and a nitric acid concentration of 0.1 M. Among the studied calixarenes, hexaacetylcalix[6]arene gave the maximum extraction efficiency of (90 ± 2)% in acidic medium. The presence of fluorinated pentadecafluoro-n-octanoic acid led to the enhancement of the extraction efficiency. The formation of a cadmium−calixarene complex was supported by energy-dispersive X-ray spectrocopy and UV−visible absorbance spectrocopy. Calixarene and its complex with cadmium were found to have distinctly different features in their scanning electron microscopy images. A solvent extraction study was carried out to study the thermodynamic aspects of complex formation and to establish the stoichiometry of the complex of the type [Cd(calixarene)]2+. The present method was utilized for the supercritical fluid extraction of cadmium from real samples of battery, paint, tobacco, and wastewater.

1. INTRODUCTION Because of the increase in population, there is an increasing demand for household products, and this process is irreversible. Different industrial activities, such as metal smelting; electroplating; mining and refining processes; and the manufacturing of cadmium−nickel batteries, plastics, alloys, and pigments, lead to the release of heavy metals such as cadmium into the environment.1−5 Some human beings are continuously exposed to cadmium through the chewing and smoking of tobacco products. Cadmium has a toxic effect on human lungs and kidneys that can eventually prove to be fatal. The major disaster of itai-itai disease in Japan was caused by cadmium toxicity. Liquid−liquid extraction is the most familiar method available for the removal of cadmium using different extracting agents. Extraction of cadmium with triphenylphosphine oxide (TPPO) by spectrophotometric method has been reported.6 In addition, cadmium was extracted from phosphoric acid medium using organophosphorous reagent by Reyes et al.7 Extractants such as alkyl xanthates,8,9 trioctylphosphine oxide (TOPO), and di-(2ethylhexyl)phosphoric acid (DEHPA)10 have been used for cadmium extraction. Selective extraction of soft metallic cadmium in HCl media by sulfur donor reagents Cynex 301 and Cynex 302 has been reported.11,12 Cadmium extractions from sulfate solution using TOPS 99 (Talcher organophosphorus solvent), PC 88A (2-ethylhexyl hydrogen 2ethylhexyl phosphonate), and Cynex 272 organophophorus extracts have been reported.13−15 However, these conventional extraction techniques are time-consuming and lead to the generation of large volumes of solvent waste.16 In recent years, supercritical fluid extraction (SFE) has become an attractive separation technique because of its inherent potential to minimize organic liquid wastes. Because of the hybrid properties of supercritical fluids (SCFs), including © 2015 American Chemical Society

liquidlike solvating ability and gaslike diffusivity, viscosity, and low surface tension, it can rapidly penetrate into matrixes.17 Solvent properties such as density and viscosity can be changed by tuning the temperature and pressure conditions of the system. Among the various SCF solvents, CO2 is widely used because of its readily attainable critical conditions (critical pressure of 72.9 atm, critical temperature of 304.3 K). It is popular because of its attractive properties such as nontoxicity, chemical stability, ecofriendliness, easy recyclability, and low cost. Direct extraction of metal ions by supercritical carbon dioxide (SC-CO2) is not possible because of the weak solute− solvent interactions and poor solubilities of metals in SCFs. To dissolve metal ions in SC-CO2, the complexation of the metal ion with an organic ligand is carried out to form a nonpolar compound that is easily soluble in the SCF. Metal ion extraction by SFE was first reported in 1992 by Laintz et al.18 βDiketone and organophosphorous reagents have been used for the SFE of actinides.19,20 Dithiocarbamate and β-diketones have been used for the SFE of Cd, Pb, and Hg from solid samples by Wai et al.21 The separation of toxic heavy metals from solid and aqueous matrixes by SFE using sulfur-containing organophosphorus reagents as chelating agents has been reported.22 Heavy metal ions have been extracted from sea sand and sewage sludge using subcritical water and supercritical CO2.23 Roa et al. extracted cadmium from sediment rocks.24 Thus, many efforts have been made to optimize different parameters for the extraction of toxic metal ions, but work has been limited to extraction parameters. However, this is the first attempt to Received: Revised: Accepted: Published: 3933

November 20, 2014 March 25, 2015 March 28, 2015 March 29, 2015 DOI: 10.1021/ie5045583 Ind. Eng. Chem. Res. 2015, 54, 3933−3941

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Industrial & Engineering Chemistry Research

the solid sample was used to study the morphology and perform a qualitative analysis of the complex. 2.2. Reagents and Materials. CO2 gas of 99.9% purity was used for the supercritical fluid. Cd(NO3)2 (AR grade, Merck, Darmstadt, Germany) was used for solution preparation. 4-(2Pyridylazo)resorcinol (PAR, S.D. Fine Chemicals) was used in the spectrophotometric determination of cadmium. AR-grade nitric acid, chloroform, and hexane were used. Buffer solution of pH 10 was prepared in Milli-Q water. 2.3. Synthesis of Calixarenes. Calixarenes were synthesized in the laboratory in accordance with a previous procedure.33,34 2.4. Procedure of SC-CO2 Extraction. Calixarene was dissolved in chloroform. The in situ mode of complexation was employed for SFE. The SFE setup that was used in the present study (obtained from JASCO, Tokyo, Japan), the detailed setup of SFE, and the procedure have been described elsewhere.35 The SFE study was performed with 1 mL (1250 μM) of ligand and 1 mL (250 μM) of metal ion solution loaded on the extraction vessel for the desired static time followed by dynamic extraction at a given temperature and pressure. The extraction vessel was then unloaded by depressurizing the system at atmospheric pressure. The unextracted part was taken in a beaker and washed with 0.1 M HNO3; then, the unextracted solution was evaporated to dryness, and the determination of cadmium was performed as described in section 2.6. 2.5. Procedure for Solvent Extraction Study. One milliliter (1250 μM) of calixarene and 1 mL (250 μM) of Cd(NO3)2 were mixed, and 2 mL of hexane was added. The solution was stirred for 45 min and then allowed to settle in a separatory funnel. The aqueous phase was evaporated to dryness and determined spectrophotometrically (section 2.6). 2.6. Determination of Cadmium Ion. 4-(2-Pyridylazo)resorcinol36 reagent was used for the spectrophotometric determination of cadmium. The liquid left in the extraction vessel after extraction was taken in a beaker, evaporated to dryness, and then dissolved in 0.1 M HNO3. The solution was filtered to eliminate organic impurities; the filtrate was made up to 5 mL with 0.1 M HNO3. One milliliter of cadmium solution was deposited in a flask, after which 1 mL of pH 10 buffer solution and 1 mL of PAR were added. The volume was then made up to 10 mL using buffer solution. The absorbance was measured at 520 nm. The cadmium concentration was calculated from the calibration graph between absorbance and cadmium amount (μg mL−1). The cadmium extraction efficiency was determined by taking the percentage ratio of the difference between the initial amount of cadmium present and the final amount of cadmium left after SFE. On the basis of a reproducibility study, a relative standard deviation of ±2% was assigned. 2.7. Effects of Other Metal Ions. A 250 μM solution of Cd2+ was taken, and known amounts of other metal ions were added. The SFE of Cd2+ with calixarenes was carried out by the proposed method. The determination of cadmium and other metal ions was carried out by atomic absorption spectrometry (AAS). 2.8. Preparation of Real Samples. The samples were prepared by the method described elsewhere37 and were spiked by adding a known concentration of Cd2+. The determination of Cd2+ was carried out by the AAS technique. 2.8.1. Tobacco. Finely ground powder of tobacco (∼5 g) was suspended in 3 mL of water and then digested with 10 mL of concentrated nitric acid at room temperature for 10 min.

report the nature of extracting complex by different characterization techniques using calixarene as a promising extractant. Calix[n]arenes are known as molecular baskets that can be formed by the condensation of phenol and formaldehyde in the presence of a base. Calix[n]arenes are a class of macrocyclic compounds that contain cavities to accommodate cations, anions, and neutral molecules.25−27 The ionophoric properties of calixarenes have been reported.28−30 Reorganization of multiple sites for ligating functionalities makes calixarenes attractive extracting agents. Calixarenes can be easily functionalized at the upper and lower rims. Calixarenes are known for their distinct physical properties such as high thermal and chemical stabilities, low toxicities, and ready modifiability. These features make them attractive for use as molecular receptors for the separation and sensing of ion and molecules.31 Because of the absence of an immune response in vivo, calixarenes have pharmaceutical applications.32 Enhancements of the binding and selectivity of the parent calixarene can be achieved by introducing different functional groups, namely, ester, ketone, amine, and nitrile groups, at the lower rims of calixarenes. Even though cadmium toxicity is a very sensitive issue, only a small number of works on cadmium extraction have been carried out by traditional solvent extraction, and very few reports exist on the SFE of cadmium. To the best of our knowledge, this is the first report on the SFE of cadmium employing calixarenes. Also, in the present study, an effort has been made not only to investigate the feasibility of SFE of cadmium using calixarenes as extractants, but also to investigate the nature of the complexes formed by the calixarenes with cadmium. The effects of various conditions on the extraction efficiency were studied, and the parameters were optimized. The influence of the addition of CO2-phillic fluorinated pentadecafluoro-n-octanoic acid (HPFOA) for the production of counterions was also examined. Supportive evidence for cadmium−calixarene complex formation was obtained by energy-dispersive X-ray spectrocopy (EDS), UV−visible absorbance spectrocopy, and scanning electron microscopy (SEM) of calixarene and its complex. Thermodynamic aspects were studied, and the stoichiometry of the complex was established by solvent extraction. The effects of the presence of a host of other metal ions were also examined. The developed method was successfully applied for the SFE of cadmium from real samples of battery, paint, tobacco, and wastewater.

2. EXPERIMENTAL SECTION 2.1. Instrumentation. Spectrophotometric analysis of cadmium(II) was carried out by UV−visible spectrometry (V670, JASCO). The metal ion concentration in aqueous solution was measured by atomic absorption spectrometry (Shimadzu-AA7000). Fourier transform infrared (FTIR) spectra were recorded using an FTIR-AR410 Bruker Tensor37 instrument as KBr pellets in the range of 4000−500 cm−1. 1 H NMR spectra were recorded on a Varian Mercury YH-300 (300 MHz) spectrometer in CDCl3 at room temperature (298 K) with tetramethylsilane (TMS) as the internal standard. The chemical composition and morphology of the complex were studied with a Leica Stereoscan-440 scanning electron microscope equipped with a Phoenix EDX attachment. The SEM images were recorded at 20 kV energy with a magnification of 5000×. The metal complex was prepared by collecting the SFE extract in chloroform and evaporating the organic phase, and 3934

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calixarene, such as the ring size and upper-rim and lower-rim functionalization. 3.1.1. Effect of Temperature on Extraction Efficiency. Temperature is an important factor influencing the extent of extraction. The influence of temperature on the extraction of Cd2+ from the aqueous phase was investigated at different temperatures from 313 to 353 K (Figure 1). Varying the

The resulting mixture was heated to dryness. After the mixture had been cooled, 1 mL of H2SO4 and HClO4 were added, and the resulting solution was heated to dryness. The final volume was made up to 25 mL with 0.1 M HNO3. 2.8.2. Paint. A ∼0.3 g sample of pink paint was taken along with 10 mL of concentrated nitric acid and then evaporated to dryness. The final volume was made up to 25 mL with 0.1 M HNO3. 2.8.3. Battery. Used battery cell powder (0.2 g) was digested with 1 mL of aqua regia solution, and then the mixture was evaporated to dryness. Water was added to the residue, which was then filtered. NH4OH and NH4Cl were added to the filtrate to precipitate Fe(III) and Al(III), and the solution was again filtered and heated to reduce the volume. Finally, the solution was made up to 25 mL with 0.1 M HNO3. 2.8.4. Water. About 10 mL of water was taken, evaporated to dryness, and then made up with 0.1 M HNO3. This solution was spiked by adding cadmium solution, and the final volume was made up to 25 mL. 2.9. Preparation of Complex. The calixarene−Cd2+ complex was prepared under ambient conditions by collecting the extracted part in an extraction vessel and used for characterization.

Figure 1. Effect of temperature on extraction efficiency (pressure, 20.27 MPa; static extraction, 30 min; dynamic extraction, 30 min; CO2 flow rate, 2 mL min−1; Cd2+ concentration, 250 μM; metal-tocalixarene mole ratio, 1:5; HNO3 concentration, 0.1 M).

3. RESULTS AND DISCUSSION 3.1. Supercritical CO2 Extraction Study. The extraction of the cadmium−calixarene complex into supercritical fluid along with the counterion in acidic 0.1 M HNO3 can be described by the equation Cd2 + + 2NO3− + nCSF ↔ [Cd2 +(C)n (NO−3 )]

temperature slightly affected the extraction efficiency. The efficiency is expected to decrease with increasing temperature because of the reduction in SC-CO2 density. Initially, the extraction efficiency increased when no volatility factor affected the effect of temperature. However, after 323 K, a steep fall in the extraction efficiency demonstrates the dominance of the density reduction. Therefore, a temperature of 323 K was selected for further study. 3.1.2. Effect of Pressure on Extraction Efficiency. The density of SC-CO2 is a function of temperature and pressure and can be predicted from the equation of state (EOS).41 At fixed temperature, the solubility is proportional to the density of SC-CO2,42,43 and solubility can be enhanced by pressure. Therefore, it is important to study the effect of pressure. In the present work, to optimize the effect of pressure at a constant temperature of 323 K, SFE was performed at pressures of 10, 15, 20, 25, and 30 MPa. The system was allowed 30 min for static extraction followed by 30 min for dynamic extraction. The variation in the extraction efficiency of Cd2+ is depicted in Figure S1 of the Supporting Information. From the observations of this pressure study, it was concluded that there is less of an effect of pressure on the extraction efficiency as the solubility of the [calixarene−Cd]2+ complex becomes saturated. Therefore, a pressure of 20.27 MPa was selected for further study. 3.1.3. Effect of Nitric Acid Concentration. The effect of the nitric acid concentration on the extraction efficiency of cadmium was studied (Figure 2). The extraction efficiency was found to increase from a nitric acid concentration 0.05 M and remain constant up to a nitric acid concentration 0.2 M. After that, the extraction efficiency decreased continuously, because NO3− serves as a counterion in acidic medium. The increase in NO3− concentration in the aqueous phase should increase the concentration of the cadmium−calixarene complex

(1)

where NO3− is the counterion and C is the calixarene. Also see Scheme 1. Scheme 1Extraction of the Cadmium−Calixarene Complex into Supercritical Fluid

The solubility of the calixarene−Cd2+ complex in the SCF medium plays a vital role in determining the extent of extraction. Chrastil38 proposed a simple empirical correlation between the solubility S of the solute and density ρ log S = k log ρ + C

(2)

where k corresponds to the number of CO2 molecules solvating the solute molecule and C is a constant term. This correlation was found to be valid for various metal complexes.39 Various parameters influence the extraction efficiency of the cadmium− calixarene complex into SCF. The effect of physical parameters, namely, pressure and temperature, on the density of an ideal gas can be correlated as described by Lin et al.40 Physical parameters such as operating pressure and temperature are expected to influence the solubility and, hence, the extraction efficiency by affecting ρ and C and the structure of the 3935

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formation of the 1:2 metal complex, which has a lower solubility because of its higher molecular weight. Thus, overall, the extraction efficiency would decrease. Therefore, in further experiments, a metal-to-calixarene mole ratio of 1:5 was employed. 3.1.5. Effect of HPFOA on Cadmium Extraction. Addition of HPFOA plays a key role in maximizing the extraction efficiency, which is well documented.45 The solubilities of fluorinated compounds in SC-CO2 are very high as compared with those of nonfluorinated compounds.46 In this case, we kept the concentrations of metal ion and ligand constant and varies the HFOA− ratio. Figure 4 indicates that the extraction

Figure 2. Effect of nitric acid concentration on cadmium extraction efficiency (pressure, 20.27 MPa; static extraction, 30 min; dynamic extraction, 30 min; CO2 flow rate, 2 mL min−1; Cd2+ concentration, 250 μM; metal-to-calixarene mole ratio, 1:5; HNO3 concentration, 0.1 M).

Cd2 +(aq) + 2NO3− + Cn(org) ↔ Cd(NO3)2 Cn(org) (3)

The decrease in extraction efficiency for higher nitric acid concentrations might be due to competitive extraction of H3O+ with calixarene. Complexation of hydronium ion with macrocyclic ligands has been reported in solvent extraction.44 Further studies were carried out in 0.1 M nitric acid medium. 3.1.4. Effect of Cadmium-to-Calixarene Mole Ratio. In our study of the metal-to-ligand mole ratio, the metal concentration was kept constant, and the ligand concentration was changed gradually to determine the effect of the ligand loading on the cadmium extraction. The extraction efficiency of cadmium initially increased up to a 1:5 ratio, where it achieved the highest percentage of extraction, but after that, it continuously decreased (Figure 3). The extraction efficiency increased with increasing metal-to-calixarene mole ratio. As is evident from eq 3, the extraction efficiency of Cd2+ is expected to increase with increasing calixarene concentration for a fixed metal amount, which was observed until metal-to-calixarene mole ratio of 1:5. Afterward, the extraction decreased, which indicates the presence of some other competing process such as the

Figure 4. Effect of HPFOA on the extraction efficiency of Cd2+ (pressure, 20.27 MPa; static extraction, 30 min; dynamic extraction, 30 min; CO2 flow rate, 2 mL min−1; Cd2+ concentration, 250 μM; metalto-calixarene mole ratio, 1:5; HNO3 concentration, 0.1 M).

efficiency of Cd2+ increased as the mole ratio of PFOA− was increased . The extraction of Cd2+ was found to depend on the PFOA − concentration. Upon the addition of PFOA− , quantitative extraction (98%) was obtained at a 1:5:20 mole ratio. However, in the absence of calixarene, the extraction of Cd2+:PFOA− was negligible. The mechanism of extraction might be as follows, where HPFOA ionizes as PFOA− and is in equilibrium with nitrate ion HPFOA ↔ H+ + PFOA−

(4)

Cd2 + + calix−NO3− ↔ Cd2 +−calix−PFOA−

(5)

The effects of variations in time (static and dynamic) were already studied and optimized by us.47 3.1.6. Multiple-Extraction Study. To maximize the extraction efficiency, we performed a multiple-extraction study. The diagrammatic representation of the multiple-extraction study is shown in Figure 5. In this case, we added 1 mL of cadmium (250 μM) and 1 mL of calixarene solution (1250 μM) in an extraction vessel. Then, the mixture was loaded in a thermostat for 30 min of static extraction at 323 K followed by 30 min of dynamic extraction, after depressurization to atmosphere level. After that, 1 mL of calixarene solution (1250 μM) was added to the unextracted part, and the extraction vessel was loaded for the same period. The unextracted solution that was left in the extraction vessel was evaporated to dryness to remove the organic impurities. The color development was performed for the determination of cadmium using the above-described method. Thus, after two cycles of extraction, it was observed

Figure 3. Effect of metal-to-ligand mole ratio on the extraction efficiency of cadmium (pressure, 20.27 MPa; static extraction, 30 min; dynamic extraction, 30 min; CO2 flow rate, 2 mL min−1; Cd2+ concentration, 25 μM; metal-to-calixarene mole ratio, 1:5; HNO3 concentration, 0.1 M). 3936

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Figure 5. Diagrammatic representation of the multiple-extraction study.

influence both the solubility and the extraction efficiency. The presence of acetyl groups in the lower position of calix[6]arene exhibits good interactions with cadmium. A calixarene with an acetyl group in the lower rim has binding sites and upper-rim functionalization with the lipophillic tert-butyl group that seem to favorably enhance the calixarene solubility and, hence, the extraction efficiency. 3.2. Solvent Extraction Study. Different calixarenes were studied for solvent extraction to determine the distribution ratio of solute. For the solvent extraction study, 1 mL of cadmium nitrate solution (250 μM in 0.1 M HNO3) and 1 mL of calixarene solution (in chloroform at a 1:5 ratio with 2 mL of hexane) were taken in a beaker and magnetically stirred for 45 min at 500 rpm. This mixture was then taken in a separatory funnel and allowed to settle for 30 min. Subsequently, the aqueous phase was separated, and cadmium was determined as described. The distribution ratio, D was defined as the ratio of concentration of cadmium in the organic phase to the concentration of cadmium left in the aqueous phase after solvent extraction. Cadmium concentration in the organic phase was, in turn, calculated from the difference between the initial concentration of cadmium in the aqueous phase and the cadmium concentration left in the aqueous phase after solvent extraction. The distribution ratios obtained after solvent extraction with different calixarenes are reported in Table 1. 3.2.1. Nature of Extracted Complex. The stoichiometry of extracted species was determined by the slope analysis method, by plotting a graph of log D versus log [C(org)] (Figure 7). The extraction was carried out by taking a fixed amount of cadmium (and a fixed nitric acid concentration) with varying

that the extraction of cadmium with hexaacetylcalix[6]arene increased from 90% to 98%. 3.1.7. SFE with Various Calixarenes. As a molecular basket, the structure of the calixarene is expected to influence the extent of formation of the calixarene−Cd2+complex (lower rim functionalization, ring size), as well as its solubility. The structures of the calixarenes used in the present study are presented in Figure 6. The highest extraction efficiency was

Figure 6. General structures of the calixarenes employed in the present study. C1: n = 4, R = H, R = OH, calix[4]arene. C2: n = 4, R = tert-butyl, R = OH, p-tert-butyl calix[4]arene. C3: n = 6, R = H, R = OH, calix[6]arene. C4: n = 6, R = tert-butyl, R = OH, p-tert-butyl calix[6]arene. C5: n = 6, R = tert-butyl, R = OAc, p-tert-butyl calix[6]arene.

Table 1. Extraction of Cd2+ with Various Calixarenesa Db

calixarene C1 C2 C3 C4 C5 a

3.56 3.45 3.13 3.15 4.62

(±0.49) (±0.14) (±0.40) (±0.32) (±0.34)

extraction efficiency by SFE (%) 72.09 74.05 74.83 75.61 90.00

(±0.33) (±2.4) (±0.55) (±2.4) (±4.4)

Results are averages of three repetitions. bLiquid−liquid extraction.

obtained for hexaacetylcalix[6]arene (Table1). According to Graham et al.’s48 study of the solubility of calixarene in SCCO2, the solubility of calix[4]arene is higher than that of calix[6]arene in SC-CO2. Therefore, it is very interesting to note that solubility is not the sole criteria for extraction, as the extent of complex formation also plays a decisive role. The roles of the presence of tert-butyl groups at the upper rim, lower-rim functionalities, and the ring sizes of calixarenes are expected to

Figure 7. log D versus log [calix]. 3937

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Industrial & Engineering Chemistry Research Table 2. Thermodynamic Functions of the Extraction of Cd2+ with Hexaacetylcalix[6]arene in Hexane Mediuma)

a

temperature (K)

log D

log Kex

ΔH (J mol−1)

ΔG° (J mol−1)

ΔS° (J mol−1 K−1)

298 301 304 307

0.6522 0.6910 0.7160 0.7795

1.6522 1.6910 1.7160 1.7795

10502.16 − − −

−9427.20 −9745.72 −9988.37 −10460.20

66.878 67.268 67.402 68.281

Cd(II), 250 μM in 0.1 M HNO3; calixarene solution, 1 mL in chloroform in a 1:5 ratio with 2 mL of hexane.

amounts of calixarene (C5), which shows the formation of a 1:1 complex with a slope of n = 0.912. A possible structure for this complex is of the type [Cd(calixarene)] 2+. From eqs 1 and 2, one can write

Kex =

D [NO3]2 [C]n

(6)

Therefore log D = log Kex + 2 log[NO3−(aq)] + n log[C(org)] (7)

3.2.2. Thermodynamic Aspects of Cadmium Extraction. Thermodynamic aspects of the extraction of Cd2+ were studied by varying the temperature from 298 to 307 K. The distribution ratio increased with increasing temperature in the given range. Figure S2 (Supporting Information) shows a graph of ln K versus 1/T. The entropy change was calculated from the slope of the graph, as ΔH ° = −slope × R

Figure 8. IR spectra showing the interactions between calixarene− Cd2+ complexes.

respectively, after complexation, indicating the interaction of Cd2+ at the lower rim of the calixarene. 3.4. Qualitative Analysis of Calixarene−Cd2+ Complex by EDS and UV−Visible Spectroscopy. UV−visible spectroscopic analysis of the calixarene−Cd2+ complex was carried out for qualitative analysis. The complexing ability of hexaacetylcalix[6]arene with cadmium was examined in a chloroform solution of ligand (1250 μM) and an aqueous solution of cadmium (250 μM) in 0.1 M nitric acid. The organic phase was used to examine the complexation. The calixarene exhibited absorption maxima at 240 and 276 nm (Figure 9). The absorption maximum at 240 nm is due to a

(8)

where R = 8.314 J/mol. The standard free energy change ΔG° and the standard entropy change ΔS at room temperature of 298 K were calculated from the relations as ΔG° = −2.303RT log Kex

(9)

and ΔS = (ΔH − ΔG)/T

(10)

The thermodynamic functions obtained for the extraction of Cd2+are listed in Table 2. The enthalpy change of extraction performed at a constant 0.1 M nitric acid concentration was evaluated as ΔH = 10502.19 J mol−1, which implies that the reaction is an endothermic process. The high negative value of the Gibbs free energy, ΔG°, indicates that the reaction is spontaneous. The spontaneity of the reaction leads to the migration of Cd2+ from the aqueous phase to the organic phase and favors the extraction of the complex. 3.3. IR and NMR Analyses. The extracted complex from SFE was subjected to IR spectroscopic analysis. There was an increase in the percentage transmittance of the calixarene peak, suggesting a decrease in the calixarene concentration. Also, the new distinct peak obtained at 754 cm−1 in the complex is probably due to the formation of Cd2+−O bond, as shown in Figure 8. An attempt was made to determine the mode of complexation between calixarene and Cd2+ by NMR spectrometry. The extracted complex obtained SFE was evaporated to dryness. The residue was dissolved in CDCl3, and this solution was used for NMR study. From the NMR spectra (Figure S3, Supporting Information), it was observed that the δ values were 3.63 and 1.89 ppm for −ArCH2Ar− and −OCOCH3, respectively, in the case of pure calixarene, but they changed to 3.55 and 1.82 ppm,

Figure 9. UV−visible spectra of calixarene and calixarene−Cd2+ complex.

π−π* transition, and the broad absorption band at 276 nm is attributed to a n−π* electronic transition. The benzene nucleus of calixarene and the π electron of the carbonyl functionality at the lower rim gives the π−π* transition, whereas the n−π* transition is due to the nonbonding electron of oxygen present at the lower rim of the calixarene. The presence of Cd2+ 3938

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Figure 10. SEM images of (a) hexaacetylcalix[6]arene and (b) calixarene−Cd2+ complex.

Figure 11. (a) TGA and (b) DTA analyses of calixarene and its complex.

623 K.49 It was observed that the calixarene and its complex started initial degredation at 620 K and were stable up to the 623 K and that only 10% weight loss occurred. The DTA curves of hexaacetylcalix[6]arene and its complex showed the effects of degradation, which might be due to the oxidation process. The maximum in the [calixarene−Cd2+] curve shows a shift to lower temperature as compared to that of calixarene. Here, we can note that the ligand has higher thermal stability than the complex. During the decomposition process, the complex degraded by breaking the Cd−O bond (formation indicated by IR spectroscopy and other techniques), resulting in an exothermic peak. 3.7. XRD Analysis of Calixarene and Its Complex. The crystalline nature of calixarene and its calixarene−Cd2+ complex was studied by XRD, and the diffractograms are shown in (Figure S5, Supporting Information). The calixarene showed a diffraction peak at a 2θ value of 30° corresponding to its crystalline phase. However, the complex showed a decrease in the intensity of this peak. This XRD pattern shows that the crystalline nature of the calixarene decreased upon complexation with Cd2+. 3.8. Effect of the Presence of Other Metal Ions. In general, cadmium is associated with other transition metal ions such as Ni2+, Pb2+, Zn2+, and Cu2+. A stock solution that was 250 μM in the metal ions was prepared in 0.1 M HNO3 and

enhanced the absorption maxima in the complex from 276 to 300 nm. This red shift in the absorption maximum due to the nonbonding electrons on the oxygen atoms of the acetyl groups indicates Cd2+−O bond formation. The EDS spectrum was measured to obtain qualitative information about the successful migration of cadmium from the aqueous layer to the organic layer (Figure S4, Supporting Information) and clearly indicates the presence of cadmium after the extraction process. 3.5. Scanning Electron Microscopy (SEM). Scanning electron microscopy (SEM) images of hexaacetylcalix[6]arene and its complex with cadmium were recorded to analyze the morphology. The sample was coated with platinum and subjected to high-vacuum conditions at room temperature. The sample was viewed at 20 kV. The SEM image confirmed the distinct morphology between calixarene and its complex (Figure 10). The calixarene had fingerlike beads that changed completely upon complexation with Cd2+, and it accumulated in various shapes. 3.6. Thermogravemetric Analysis. The results of the thermogravemetric analysis of calixarene and [calixarene−Cd2+] complex are shown in Figure 11. The thermal analysis was carried out in the range of 303−1073 K in air atmosphere at a flow rate of 50 mL/min using alumina as the reference material. Hexaacetylcalix[6]arene is stable in the range between 593 to 3939

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Industrial & Engineering Chemistry Research extracted with hexaacetylcalix[6]arene. The aqueous layer was subjected to the determination of the metal ions by atomic absorption spectroscopy (AAS). Zn2+ exhibited less than 40% extraction, whereas other ions were extracted up to 65%. To demonstrate selectivity in extraction, cadmium was successfully extracted from a Cd−Zn alloy.

a

battery paint tobacco water

2.234 2.309 2.019 2.294

extraction efficiency of Cd2+ (%) 89.83 91.74 93.55 92.54

± ± ± ±

0.48 0.55 0.38 0.24

In 25 mL.

5. CONCLUSIONS Cadmium, a toxic heavy metal, could be successfully extracted by SFE employing hexaacetylcalix[6]arene. The maximum extraction efficiency of (90 ± 2)% was obtained for 30 min of static extraction followed by 30 min of dynamic extraction, at a pressure of 20.27 MPa, a temperature of 323 K, a CO2 flow rate of 2 mL min−1, a metal-to-calixarene mole ratio of 1:5, and a HNO3 concentration of 0.1 M. The presence of HPFOA enhanced the extraction efficiency. A solvent extraction study with hexane as the organic phase indicated the formation of [Cd−calixarene]2+ complex. Complex formation was ascertained with the help of UV−visible and EDS spectra. The suitability of the developed method was examined in the presence of a host of other metal ions, and the method was finally applied to the SFE of Cd2+ from real samples (namely, battery, paint, tobacco, and industrial waste released in water).

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ASSOCIATED CONTENT

S Supporting Information *

Effects of pressure and temperature on the extraction of Cd2+; NMR, EDS, and XRD spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

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REFERENCES

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Table 3. Extraction Efficiency of Cd2+ from Real Samples Cd presenta (ppm)

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The authors acknowledge the keen interest and support of Dr. B. S. Tomar, Head, RACD, BARC. N.V.R. and D.D.M. thank the Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, Mumbai, India, for financial support (No.2011/37C/02/BRNS). Acknowledgement is due to the Department of Chemistry, Savitribai Phule Pune University, for providing IR, NMR, AAS, TGA, DTA, and XRD facilities during our research.

4. APPLICATIONS To determine its applicability, this method was utilized for the analysis of various real samples including tobacco, paint, battery, and water. These products are responsible for the pollution of the environment. After decomposition of these products, cadmium waste is released into the environment. A literature review showed that many people have suffered from cancer from tobacco chewing. Small children are at risk of cadmium toxicity from the handling of crayons, paints, plastic toys, and so on. With this in mind, we established a method for extracting toxic heavy metal Cd2+ from these samples. The stock solution was prepared as described in section 2.8. The results (Table 3) show that cadmium can be separated from real samples efficiently.

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Corresponding Author

*Tel.: +91-20-25693981. Fax: +91-20-25691728. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3940

DOI: 10.1021/ie5045583 Ind. Eng. Chem. Res. 2015, 54, 3933−3941

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Industrial & Engineering Chemistry Research

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