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 Table of Contents 
Year : 2013  |  Volume : 36  |  Issue : 2  |  Page : 78-84  

Para-tert-butylcalix[4]arene as promising complexing agent for removal of the strontium from the aqueous medium

1 Division of CBRN Defence, Institute of Nuclear Medicine and Allied Sciences, Delhi; Department of Pharmaceutics, JSS College of Pharmacy, JSS University, Mysore, Karnataka, India
2 Division of CBRN Defence, Institute of Nuclear Medicine and Allied Sciences, Delhi, India
3 Department of Pharmaceutics, JSS College of Pharmacy, JSS University, Mysore, Karnataka, India

Date of Web Publication14-Mar-2014

Correspondence Address:
Rakesh Kumar Sharma
Division of CBRN Defence, Institute of Nuclear Medicine and Allied Sciences, Brig. S. K. Mazumdar Road, New Delhi - 110 054
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Source of Support: Director, INMAS Delhi India for providing experimental facilities and the financial support for this work., Conflict of Interest: None

DOI: 10.4103/0972-0464.128873

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Strontium-90 is one of radioactive nuclear fallout products, can cause serious health effects. Efficient techniques are needed to remove radioactive strontium from contaminated persons. In this study complexation properties of the p-tert-butylcalix[4]arene has been evaluated against cold strontium nitrate. Ultra violet-visible (UV-vis) and fluorescence spectrophotometric techniques were used for the qualitative analytical screening and the inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique used for the quantitative complexation of p-tert-butylcalix[4]arene with strontium nitrate. UV-vis, fluorescence spectroscopy and ICP-AES studies confirmed the complexation of p-tert-butylcalix[4]arene and strontium. Extraction study of strontium from picric acid by trans-chelation method proves that p-tert-butylcalix[4]arene is nearly 87% ± 3% effective. Complexation can be attributed to the cation-lone pair interaction and the bonding between the Sr 2+ and the hydroxyl group of the p-tert-butylcalix[4]arene implying its promise as a complexing agent for the removal of strontium.

Keywords: Inductively coupled plasma-atomic emission spectrometry, p-tert-butylcalix[4]arene, spectrofluorimetry, strontium, transchelation

How to cite this article:
Sharma N, Rana S, Shivkumar HG, Sharma RK. Para-tert-butylcalix[4]arene as promising complexing agent for removal of the strontium from the aqueous medium. Radiat Prot Environ 2013;36:78-84

How to cite this URL:
Sharma N, Rana S, Shivkumar HG, Sharma RK. Para-tert-butylcalix[4]arene as promising complexing agent for removal of the strontium from the aqueous medium. Radiat Prot Environ [serial online] 2013 [cited 2023 Mar 31];36:78-84. Available from: https://www.rpe.org.in/text.asp?2013/36/2/78/128873

  Introduction Top

Uncontrolled chain reaction during nuclear reactor accident or explosion of nuclear devices results in the release of a number of radionuclides especially long-lived fission products, such as Cesium-137 ( 137 Cs), Iodine-131 ( 131 I), strontium-90 ( 90 Sr) and other gaseous fission products into the air. Due to their long half-lives, they remain into the environment as well as in the exposed living systems for many years. Their radioactive decay produces high energy beta particles and gamma ionizing radiation that can cause localized radiation injury. 90 Sr is a beta emitter and has a relatively long half-life of about 28.8 years. [1] Chernobyl nuclear reactor accident [2] and Fukushima nuclear reactor accident [3] revalidate the importance of preventing the intake of the various fallout products and the possible health effects.

Selective detection, quantification and extraction of radioactive metal ions are of great importance not only for the efficient medical management, but also in chemical, biological and environmental sciences. [4],[5],[6],[7] 90 Sr is one of the major beta emitter radionuclide present in the high level waste (HLW). [8] Inhalation is the most important pathway along with, absorption or translocation of radionuclides can also take place through the contaminated skin. [9] Once radio-isotopes reaches systemic circulations it tends to accumulate in the specific target organs. 90 Sr preferentially gets deposited into the bone and its relatively long half-life make it one of the more hazardous radionuclides present in the fallout. [10]

Calixarene can bind efficiently with smaller metal or ions [11],[12] and has been used for solvent extraction, development of many extractants, transporters, stationary phases, electrode ionophores and electrochemical sensors over the past four decades. [13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23] Calixarenes are synthetic macrocyclic compound that belongs to the third generation of the host molecules and are generally used as building blocks for the accurate arrangement of binding sites in space, which co-operatively display high selectivity for the substrates. They have a variable number of reactive positions for attaching the ligating functions and well-defined cone conformations capable of forming inclusion complexes. [24] The hydrophobic apolar cavities eventually enable to encapsulate ions with small steric demands. [25]

Calixarenes have been extensively studied for a wide range of metal ions, such as silver, gold, nickel, cobalt and copper due to its selective complexation, sensing and transport. [26] To use metal complexation property of the calixarenes, nanoemulsion has already been developed and studied for the uranium extraction from contaminated uranyl ion solution. [27] The present study, reported the qualitative analytical screening of the complexation of p-tert-butylcalix[4]arene [Figure 1] toward Sr 2+ (strontium nitrate) by ultraviolet visible (UV-vis), fluorescence technique and quantitative analytical screening of the complexation with the help of inductively coupled plasma-atomic emission spectrometry (ICP-AES).
Figure 1: General molecular structure of p-tert-butylcalix[4]arenes

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The in vitro extraction efficiency of p-tert-butylcalix[4]arene was evaluated with the help of Trans-chelation reaction method by using strontium picrate in extraction experiment. [28],[29],[30] p-tert-butylcalix[4]arene holds promise not only provide a low cost and easily available complexing agent for the removal of Radioactive-heavy metals such as 90 Sr but also shows great potential to extract most of the strontium metal ions from the solution.

  Materials and Methods Top


Extra pure p-tert-butylcalix[4]arene was a gift sample from Neelkanth Distributors, Vadodara, Gujarat. Strontium nitrate, dimethyl sulfoxide (DMSO) high-performance liquid chromatography (HPLC) grade 99.9%, tetrahydrofuran (THF) HPLC spectrophotometric grade 99.9%, acetonitrile HPLC grade 99.93% and chloroform HPLC grade 99.9% were purchased from Sigma-Aldrich Honeywell Int. Inc. Germany. All aqueous solutions were prepared in deionized water that had been passed through a Millipore milli-Q Plus water purification system Germany.


UV-vis spectra were recorded on a Double Beam UV-vis Spectrophotometer UV5704SS (Electronic Corporation of India Limited, Hyderabad, India) using standard 1.00 cm quartz cells. Fluorescence study was performed on Spectramax M 2 Molecular Device luminescence spectrometer (Sunnyvale, CA, USA) using SoftMax Pro Software, Sunnyvale, California, United States, (Hz-6621) mechanical shaker with temperature controller (Loughborough, UK) was used for shaking.

ICP-AES instrumentation

Strontium concentration was determined using ICP-AES (Optima 2000 DV, Perkin Elmer, CT, USA) and New Wave Laser Ablation unit (UP-213 (Nd-YAG), Cambridge, UK). Argon was used for the laser cell as the plasma carrier gas.

Analytical methods

Method for solvatochromic behavior

Solubility studies of the p-tert-butylcalix[4]arene [31] were performed in different solvents such as acetonitrile, THF, chloroform and dimethylsulfoxide. Significant changes in the UV-vis spectra were recorded on addition of p-tert-butylcalix[4]arene to the different solvents, while the p-tert-butylcalix[4]arene concentration was kept constant in all experiments. 18 mg of calixarene was added to 10 ml of different solvents and the mixture was shaken in thermostatic water bath shaker for 1 h at 25°C. The solution was checked visually through naked eye for the undissolved salt, after 1 h shaking, no undissolved salt has been found which shows that 1 h time is sufficient for the solution to reach at equilibrium. The solution absorbance was determined by UV-vis spectrophotometer. To find stoichiometric ratio between the p-tert-butylcalix[4]arene and strontium, method of continuous variation (Job's method) [28] was applied by plotting absorbance versus mole fraction. p-tert-butylcalix[4]arene (10 -4 M) and strontium nitrate (10−4 M) was dissolved in chloroform with stirring for 1.5 h. Any change in the UV-vis spectra of calixarene was recorded by the addition of strontium nitrate in 1:1 ratio with that of calixarene.

Method for ICP-AES

Preparation of samples

Samples for the complexation studies of p-tert-butylcalix[4]arene and strontium were prepared with the help of the Franz cell diffusion technique at a time interval of 0 h, 1 h, 2 h and 4 h respectively. [32] The initial concentration of strontium was 2 μg/ml and the final concentration of strontium in the solution was measured at the previously mentioned time intervals with the help of ICP-AES.

Preparation of standards

Calibration standard was prepared by diluting a standard stock of 1 ppm of strontium (Spex Certiprep Standards, Metuchen, NJ, USA) and working standards in 2% (v/v) sub-boiled nitric acid (Optima Grade, Fishier Scientific, Pittsburgh, PA, USA). The ICP-AES operating condition and wavelength used are shown in [Table 1].
Table 1: ICP - AES operating parameters

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These operating conditions were selected after optimization of each parameter. The concentration of calibration standard solutions for ICP-AES was zero (ultra-pure water), 0.005, 0.02, 0.1, 0.5, 1 and 2 μg/L, for generation of calibration curves, determination of each concentration was repeated 3 times.

Extraction using picric acid

Trans-chelation technique had been followed for the extraction of metal ions. [28],[29],[30] For this Strontium picrate solution (2.5 × 10 -5 M) was prepared by adding 1 × 10 -2 M of strontium nitrate solution to 2.5 × 10 -5 M aqueous picric acid solution at 25°C and shaken for 1 h, 5 mL (V) of strontium picrate solution (2.5 × 10 -5 M) and 5 mL of p-tert-butylcalix[4]arene solution (1 × 10 -3 M or 0.064 g (W) in 100 ml of chloroform) were vigorously agitated in a stopper flask with a mechanical shaker for 2 min. The mixture was magnetically stirred at 25°C for 1 h and finally left standing for an additional 30 min. Then spectrophotometric technique was used to determine the concentration of picrate ion remaining in the aqueous phase. A similar extraction was performed in the absence of p-tert-butylcalix[4]arene in aqueous solutions. The amount of metal cation extracted by p-tert-butylcalix[4]arene (q) was calculated as:

where n is valence of metal ion

The extractability of the strontium cations (E s ) is expressed by means of the Eq. (2):

Where Ci and C are initial and final concentrations of strontium before and after the extraction respectively.

  Results and Discussion Top

Supramolecular chemistry provides a novel approach for the complexation and removal of the various metal ions. Calixarene are an important class of macrocyclic compounds which not only act as a base moiety for complexing agent but also emerge as a base plateform for synthesizing new and efficient ligands. Selective fictionalizations of both upper and lower rims of calixarene podands make calixarene efficient ionophores, to extract or bind target metal ions selectively. [33] It has been successfully shown efficacy for complexing strontium nitrate after extraction from picric acid followed by UV-vis, fluorescence and ICP-AES studies. The efforts made to evaluate and explore qualitative analytical screening and the strontium extraction ability of p-tert-butylcalix[4]arene is discussed below.

Solvatochromic behavior

Variation in the UV-vis spectra of p-tert-butylcalix[4]arene in terms of position, intensity and shape of the absorbance bands in various solvents revealed that solvent plays an important role in complexation process. [34] Study of solvatochromic effect helps in selecting the solvent for a particular ligand that could not show similar spectra in various solvents. In particular, calixarene based ligands show aggregations in some solvents depending on the solvent-solute interactions. In order to examine these effects on metal complexation of p-tert-butylcalix[4]arene, a series of solvents were used and analyzed by UV-vis spectrophotometry. It has been observed that solvents like acetonitrile (CH 3 CN), THF, chloroform (CHCl 3 ) and DMSO offer good results. In THF and CHCl 3 , p-tert-butylcalix[4]arene shows significantly different spectral results in terms of disappearance of its aggregated bands and appearance of more intense bands with greater shifts in λmax as compared with DMSO as shown in [Figure 2].
Figure 2: Absorption spectra of ligand in different solvents (5 × 10-5 M). (a) Acetonitrile, (b) tetrahydrofuran, (c) chloroform, (d) dimethylsulfoxide

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This is the well-known fact that with the increase in polarity of the solvent the absorption bands shift toward longer wavelengths (i.e., positive solvatochromism). This effect results due to the presence of non-bonding electron pairs of carboxylic groups in the p-tert-butylcalix[4]arene. In case of p-tert-butylcalix[4]arene, appreciable absorption bands are seen in the region 220-250 nm through n→π* transitions. Many attempts have been made to observe the solvatochromic behavior of p-tert-butylcalix[4]arene in different solvents such as methanol, ethanol, acetone and dimethylformamide was studied even at low concentration; but due to aggregation and too much noise in the spectra, they have been decided as superfluous except in CHCl 3 and THF. Thus, on the basis of stability, solubility of nitrate salts of transition metals and to ascertain effective complexation without the formation of aggregates CHCl 3 was selected as best solvent for complexation study.

Complexation studies

Analytical procedure of UV-vis, fluorescence measurements to evaluate the complexation behavior of p-tert-butylcalix[4]arene.

UV-spectroscopic studies

The chromogenic and ionophoric properties of p-tert-butylcalix[4]arene were evaluated by examining the UV-vis absorption spectra of p-tert-butylcalix[4]arene in chloroform. After complexation, pronounced changes in the absorption spectra were observed which shows that p-tert-butylcalix[4]arene has remarkable affinity toward strontium metal ion.

[Figure 3]a and b represents that there is the significance decrease in the % (percentage) absorbance from 2.27 of p-tert-butylcalix[4]arene to 0.55 in p-tert-butylcalix[4]arene -Sr 2+ complex at 280 nm. The bands show complexation ability of p-tert-butylcalix[4]arene toward strontium metal ion. It has been proposed that the change is attributed to the complexation of strontium metal ion with the four binding sites arranged three dimensionally on the calix framework.
Figure 3: Ultra violet-visible spectra of ligant with strontium ion in chloroform: (a) Blank, (b) ligant with Sr2+

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Fluorescence-spectroscopic studies

Due to the highest UV-vis absorbance responses of p-tert-butylcalix[4]arene with strontium metal, it has been decided to explore its fluorescence sensing behavior too. As shown in [Figure 4] there is a large and highly selective fluorescent quenching effect in the emission spectrum of p-tert-butylcalix[4]arene with Sr 2+ . The studies were carried out in chloroform solution of p-tert-butylcalix[4]arene with strontium nitrate. Fluorescence spectrum of p-tert-butylcalix[4]arene shows enhancement of fluorescence intensity in terms of relative fluorescence units (rfu) from 110 to 160. The fluorescence spectrum of p-tert-butylcalix[4]arene exhibited a typical emission band at 375 nm. Upon addition of strontium nitrate into the solution of p-tert-butylcalix[4]arene, a significant change was observed in the 375 nm emission band. Beside this a new red-shift emission band centered at 380 nm was also appeared as a shoulder in the previous band with high intensity.
Figure 4: Fluorescence emission spectra of ligant and its complex with Sr 2+max = 380 nm)

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The strong red-shift emission for the p-tert-butylcalix[4]arene with Sr 2+ can be attributed to the cation-lone pair interaction and the coordination bonding between the Sr 2+ and the hydroxyl group of the p-tert-butylcalix[4]arene as proposed in [Scheme 1].

ICP-AES studies

Strontium concentration in the solution was found for each one of the sample solutions A1, A2, A3, B1, B2, B3 and C1, C2, C3 the initial concentration of strontium was 2 μg/ml, 5 μg/ml and 7 μg/ml respectively and the concentration of the strontium in the permeated samples after the time intervals of 1 h, 2 h and 4 h the final concentration was found to be less than 1 μg/ml. Results as have been tabulated in [Table 2].
Table 2: The initial and final concentration of the sample solutions A1, A2, A3, B1, B2, B3 and C1, C2, C3 at 0 h, 1 h, 2 h and 4 h respectively

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Extraction study

Trans-chelation method for the extraction of Sr 2+ ion was followed to determine the efficiency of the p-tert-butylcalix[4]arene to remove the metal picrate (strontium picrate) from the aqueous phase. Solution of p-tert-butylcalix[4]arene in chloroform was used to remove the strontium picrate from the aqueous phase then the concentration of the picrate in the aqueous phase has been determined spectrophotometrically. The extraction capacity and extraction percentage were calculated using Eq. (1) and (2), and given in [Figure 5] and [Figure 6]. These data have been obtained by using a 0.064 g of the p-tert-butylcalix[4]arene for extraction of metal cations from aqueous solution.
Figure 5: Extraction capacity (q) of blank and ligand towards Sr+2 (25°C, 1 h, Ci = 0.02, mmol L-1)

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Figure 6: Extraction study of ligand with strontium picrate a,b ( a strontium picrate solution = 2.5 × 10 -5 M and ligand solution = 1 × 10 -3 M, b averages and standard deviations calculated for data obtained from three independent extraction experiments)

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[Figure 6] shows the extraction into chloroform at a fixed concentration of the p-tert-butylcalix[4]arene for Sr +2 which show the 87% ± 3% extractability as compared to its blank solution which contains only chloroform solution.

From the data given in [Figure 5] and [Figure 6], it has been observed that p-tert-butylcalix[4]arene shows great affinity toward Sr +2 .

  Conclusion Top

p -tert-butylcalix[4]arene has the great potential to extract the Sr 2+ metal ion from the solution. This can be supported by the qualitative and quantitative analytical screening and complexation of the p-tert-butylcalix[4]arene with the help of the UV-vis, fluorescence spectroscopic and ICP-AES techniques. The p-tert-butylcalix[4]arene shows clear indication of complexation with strontium. Secondly, Strontium removal properties were investigated with the help of Trans-chelation method. The p-tert-butylcalix[4]arene was observed to have extraction efficiency of 87% ± 3% toward strontium. The specificity of strontium can be attributed to the co-ordination bonding taking place between the Sr 2+ ion and the Hydroxyl group present at the inside rim of p-tert-butylcalix[4]arene. Thus, the p-tert-butylcalix[4]arene appears promising agent for the complexation of the Sr 2+ ion and thus can be used for decontamination of the activity from aqueous solutions.

  Acknowledgments Top

Author thank JSS College of Pharmacy, Mysore, Karnataka India and Director, INMAS Delhi India for providing experimental facilities and the financial support for this work.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]

  [Table 1], [Table 2]

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