Synthetic Resin Amberlite Xad 4 Biology Essay

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Synthetic resin, Amberlite XAD-4 was linked covalently with the third generation supramolecule, octa-O-methoxy resorcin arene through -N=N- group to form chelating resin which has been characterized and effectively used for the separation and preconcentration of metal ions such as Ni(II), Cu(II), Zn(II) and Cd(II). Critical parameters such as pH, flow rate, sorption capacity, breakthrough studies, distribution coefficient, preconcentration factor, concentration of eluting agents responsible for quantitative extraction of metal ions were optimized. The synthesized resin showed good binding affinity towards Ni(II), Cu(II), Zn(II) and Cd(II) under selective pH conditions. Good breakthrough capacity and fast exchange kinetics of the resin lead to effective separation of metal ions from their binary and ternary mixture by column method on the basis of pH and eluting agents. The resin could be reused for about 8-10 cycles. The proposed method having the analytical data with the relative standard deviation (RSD) < 2% and with recoveries of analytes higher than 98%, reflect upon the reproducibility and reliability of the method which has been successfully applied for the separation and determination of Ni(II), Cu(II), Zn(II) and Cd(II) ions in synthetic, natural and ground water samples.

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Keywords: Separation; Preconcentration; Amberlite XAD-4; Octa-O-methoxy resorcin[4]arene; Solid phase extraction

Introduction

It is well recognized that several trace elements are essential constituents of enzymes and play a vital role in human metabolism and primarily supplied through diet and drinking water. However, beyond the essential range, deficiency and toxic effects are observed

[1]{Daneshfar, 2012 #2}. Considering biological research, the role of some trace and ultra-trace elements in the body is very important and has diverse functions. Some of the trace elements are essential to life while others are toxic even at low concentrations [2-6]. Many metals listed as environmental hazards are essential dietary trace elements required for normal growth and development of animals and human beings. These metals are essential to human life within permissible limits. Toxicity of metal ions such as nickel, copper, zinc and cadmium in human beings is as follows:

Nickel: Nickel plays important roles in the biology of microorganisms and plants [7-12]. Nickel sulfide fume and dust is believed to be carcinogenic, various other nickel compounds may be as well [7-12].

Copper: Copper is an economically important element which is found only in trace quantities in the earth's crust. It is required as a trace nutrient for both plants and animals, but excessive amounts are toxic [13].

Zinc: Men and many animals exhibit considerable tolerance to high zinc intakes. This tolerance is dependent on the nature of diet, and its Ca, Cu, Fe and Cd contents with which zinc interacts in the process of adsorption and utilization. Symptoms of zinc toxicity in humans include vomiting, dehydration, electrolyte imbalance, abdominal pain, nausea, lethargy, dizziness and lack of muscular disco-ordination [7-12]

Cadmium: Cadmium is non-essential and toxic to human and animal systems [14]. Small quantities of cadmium cause adverse changes in the arteries of human kidneys and liver[15]. Teratogenic properties have been shown [16] where as carcinogenic properties are suspected [17-19].

In this regard, evaluation of trace metals in the environmental samples including natural water has been continuously performed to designate the level of pollution which creates health problems [20, 21]. Several analytical techniques have been used for the determination of heavy metal ions in the real samples which include Inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) [22], flame atomic absorption spectrometry (FAAS) [23] etc. These techniques are both widely and routinely used for the determination of trace amounts of heavy metal ions. However, the direct determination of heavy metal ions at trace levels is limited not only due to insufficient sensitivity, but also by matrix interference [24, 25]

In order to overcome these difficulties, preliminary separation and preconcentration of trace elements from the matrix are frequently used techniques to improve the detection limit and the selectivity [26]. Separation technique, in general, deal with the separation of components of mixtures to enhance the purity of substances while in preconcentration technique, one of the analytes or species have been selectively retained on a solid phase and then eluted with an appropriate eluent, and detected and quantified by an appropriate detection technique [27].

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Different separation and preconcentration techniques using solid phase extraction (SPE) [28], co-precipitation[29] solvent extraction [30], membrane filtration [31], cloud point extraction[32, 33], flotation[34], electrochemical decomposition [35] ion exchange etc. have been developed for the trace metal ion determination. Among these solid-phase extraction technique have been widely used because of its various advantages over other methods, such as higher preconcentration factor, lesser waste generation, lower matrix effect, use of less toxic solvents, saving of time and cost, easy regenerability of a solid phase and thereby more reusability [36-39].

Various SPE materials which have been used for the preconcentration of trace metal ions as their chelates include, activated silica gel [40, 41], carbon [42-45], polyurethane foam [46] microcrystalline naphthalene [47], C18 cartridges [48], Chelex-100 [49], Alumina[50], functionalized polyurethane foam [51], modified silica [52, 53], sulfur powder [54] and Amberlite XAD resins[55-61]. Amberlite XAD (styrene-divinyl benzene copolymer) resins, as the copolymer backbone for the -immobilization of chelating ligands, have some physical superiority, such as porosity, uniform pore size distribution, high surface area, and chemical stability toward acids, bases, and oxidizing agents, as compared to other resins [62-65]. Either the adsorption of chelating ligands onto these supports or the covalent coupling of chelating moiety has been used to design polymeric chelating resins, which are versatile, durable, have good loading capacity towards metals, exhibit enhanced hydrophilicity and flexible working conditions. That is why the reaction of Amberlite XAD resin with suitable chelating agents is very popular for separation and preconcentration [66-71].

Calix[4]resorcinarenes possess good capability to form a variety of complexes with organic/inorganic ions, neutral molecules, transition metal complexes and organometallics [72] for the simple reason that they can be functionalized at the upper rim or at C-methyl position or at hydroxyl groups at extra-annular position, enabling them to be used as multifunctional receptors[73]. Literature reveals only few reports on the extraction of metal ions using calix[4]resorcinarene and its derivatives [74-80]. As far as work on polymer supported calix[4]resorcinarenes is concerned, Merdivan et al. [31, 81, 82] have reported the use of calix[4]resorcinarene impregnated on Amberlite XAD for separation and preconcentration of rare earths.

Here, the use octa-O-methoxy resorcin[4]arene Amberlite XAD-4 chelating resin for extraction, preconcentration and sequential separation of metal ions such as Ni(II), Cu(II), Zn(II) and Cd(II) in a column prior to their determination by Spectrophotometry/FAAS/ICP-AES is described. Various factors influencing the separation and preconcentration of these metal ions, such as pH, concentration of eluting agents, flow rate, total sorption capacity, exchange kinetics, preconcentration factor, distribution coefficient, breakthrough capacity, resin stability, effect of electrolytes, and associated metal ions have been investigated. The newly developed method has also been applied for the determination of Ni(II), Cu(II), Zn(II) and Cd(II) metal ions from industrial affluent, natural river and ground water samples of Ahmedabad (Gujarat) city.

Experimental

2.1 Reagents

All AR grade reagents purchased from Sigma-Aldrich and Merck were used for preparations of the standard and sample solutions. Amberlite XAD-4 with surface area 750 m2 g−1, pore diameter 50Å and bead size 20-50 mesh was procured from Fluka All aqueous solutions were prepared with quartz distilled deionized water. Glassware were soaked in 5% HNO3 overnight before use and cleaned repeatedly with double distilled deionized water. The pH was adjusted with the following buffer solutions: PO4-3/HPO4-2 buffer for pH 2.0 and 3.0; CH3COO-1/ CH3COOH buffer for pH 4.0 to 6.0; HPO4-2/H2PO4-1 buffers for pH 7.0 and 7.5; NH3/NH4+ buffers of pH 8 to 10.

Standard stock solutions (1000 μg mL-1) of Ni (II), Cu (II), Zn (II), and Cd(II) were prepared by dissolving 0.4045 gm NiCl2.6H2O; 0.384 gm Cu(NO3)2.3H2O; 0.4541 gm Zn(NO3)2.6H2O; 0.2744 gm Cd(NO3)2.4H2O in 1 mL concentrated HNO3 and dilute upto 100 mL with water in volumetric flask.

2.2 Instruments

The metal content determinations were carried out using (FAAS) NOVAA 400p model with hollow cathode lamps, equipped with an air-acetylene flame without background correction and the operating conditions adjusted were carried out according to the standard guidelines of the manufacturers. Spectral measurements were carried out using JASCO 570 UV-vis spectrophotometer using 10mm quartz cells. Inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) JY 2000-2 model with the plasma scan multitasking computer and a peristaltic pump was used under optimum working conditions: concentric glass nebulizer, cyclonic glass spray chamber, 3 channels peristaltic pump, thermo-regulated, 0.64 meter focal length, 2400 g/mm grating used in the first order with optical resolution <19 pm for 160-800 nm (optional far UV for extended wavelength range 120-800nm), 12 L/min plasma gas, 0.2 L/min sheath gas. The flow of the liquid through the column was controlled by Miclins Peristaltic pump PP-10 EX. Elico digital pH-meter, model L1 614 equipped with a combined pH electrode was employed for measuring pH.

2.3 Synthesis

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Octa-O-methoxy resorcin[4]arene, the third generation supramolecule was synthesized by the acid catalyzed condensation reaction of 1,3 dimethoxy benzene and p-hydroxy benzaldehyde as reported earlier [83]. Octa-O-methoxy resorcin[4]arene covalently linked with Amberlite XAD-4 through azo (-N=N-) linkage as per Scheme 1.

2.3.1 Synthesis of Amberlite XAD-4 -octa-O-methoxy resorcin[4]arene

The nitration of Amberlite XAD-4 beads, the reduction of the nitrated resin and the subsequent diazotization of the amino resin was carried out by the procedure reported earlier for Amberlite XAD-2 [84]. The resulting diazotized resin was allowed to react with octa-O-methoxy resorcin[4]arene (1.0 g dissolved in 50mL of 10% NaOH) at 0-5 °C for 48 h. The dark brown coloured beads were filtered, successively washed with double distilled water and finally dried for further studies.

2.4 Recommended column method for separation and preconcentration of metal ions

1.0 gm of octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric chelating resin was added to the mixture of CH3OH:H2O (1:1) to get a slurry and poured onto the (10 cm long, 1 cm inner diameter) glass column equipped with a stopcock and a porous disk. Which was then washed with deionized water and conditioned with 15-20 mL of buffer solution of desired pH of the sample solution containing metal ions (Ni(II),Cu(II),Zn(II) and Cd(II)) at an optimum flow rate. The amount bound metal ions stripped with suitable eluting agents was determined by spectrophotometry/FAAS/ICP-AES.

2.5 Recommended batch method for preconcentration and determination of metal ions

Octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric chelating resin (0.5 gm) was added into a glass stoppered bottle containing 100 mL of Ni (II)/Cu(II)/Zn(II)/Cd(II) solutions at an optimum pH. The bottle was shaken for about 30 minute and the resin was filtered. Using the suitable eluting agents, the amount of metal ions in resin was determined by spectrophotometry/FAAS/ICP-AES.

Results and discussion

3.1 Spectral interpretation of synthesized octa-O-methoxy resorcine[4]arene Amberlite XAD-4 polymeric chelating resin.

The octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric cheltaing resin was characterized by elemental analysis, FT-IR and mass difference. The nitrogen content in Amino-XAD-4 was found to be 2.73% higher than Nitro-XAD-4, confirms the successful reduction of Nitro-XAD-4 resin. The FT-IR spectra of NO2- XAD-4, NH2-XAD-4 and octa-O-methoxy resorcin[4]arene Amberlite XAD-4 chelating resin, are given in Fig. 1 Asymm (N-O) and symm (N-O) stretching bands of the NO2- XAD-4 were observed at1540 and 1325 cm-1, respectively. The N-H stretching vibrations of NH2-XAD-4 were identified with the bands at 3400 and 1625. The conspicuous band of -N=N- at 1470 cm-1 confirms the successful formation of octa-O-methoxy resorcin[4]arene Amberlite XAD-4 chelating resin through -N=N- linkage. Apart from this, the loading of octa-O-methoxy resorcin[4]arene (0.61 mmole g-1) on synthetic XAD-4 resin was calculated by mass difference of dried resin, which also confirms nitration, reduction and coupling of diazotized Amberlite XAD-4 resin with octa-O-methoxy resorcin[4]arene.

3.2 Parameters optimized for separation and preconcentration of Ni(II), Cu(II), Zn(II) and Cd(II)

For quantitative recoveries of Ni(II), Cu(II), Zn(II) and Cd(II) with the octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric chelating resin, various parameters like pH, flow rate, concentration, type and volume of the eluting agents, sorption capacity, distribution coefficient (Kd), exchange kinetics, breakthrough studies, preconcentration factor and reusability of the resin were evaluated.

3.2.1 Effect of pH on quantitative absorption

The initial pH of the solution is most important parameter used for sorption studies. The influence of the pH of the aqueous solution containing 5 µg mL-1 of each Ni(II), Cu(II), Zn(II), and Cd(II) on 1.0 gm of octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric chelating resin was investigated using the batch method. A 100 mL aqueous solution of each metal ion with buffer solution of desired pH was placed in glass stoppered bottles stirred slowly for 1 hour. The pH for maximum sorption of Ni(II), Cu(II), Zn(II), and Cd(II) was found to be 3.5, 5.5, 5.0 and 8.0, respectively (Table 1, Fig. 2).

3.2.2 Effect of flow rate on metal sorption

The sorption of metal ion on resin (1.0 gm) in a packed column was studied at various flow rates. Feed solutions containing 5.0 μg mL-1 of Ni(II), Cu(II), Zn(II) and Cd(II) were passed through the column at different flow rates (0.5, 1.0, 1.5, 2.0, 2.5 etc., mL min-1), maintained by a peristaltic pump. Optimum flow rate may be defined as the rate of flow of the effluent through the column at which more than 98% sorption takes place. The optimum flow rates obtained for resin were 2.5, 2.0, 1.5 and 1.0 ml min1 for Ni(II), Cu(II), Zn(II), and Cd(II), respectively (Table 1). It was observed (Fig. 3) that, as the flow rate increases the sorption decreases, because the time required for the metal ion to come in contact with the chelating resin is less, therefore the sorption of metal ion decreases.

3.2.3 Effect of concentration of eluting agents

The choice of a suitable eluent is another important factor to obtain efficient and selective recovery of analytes. In this study, a series of experiments were performed to obtain a reasonable eluent to elute Ni(II), Cu(II), Zn(II), and Cd(II) ions from metal bound resin. 1.0 gm resin in the column was conditioned at pH of maximum sorption and then fed with 100 mL solutions containing 5.0 μg mL-1 metal ions (Ni(II), Cu(II), Zn(II), and Cd(II)). The metal ions were desorbed with different concentrations of acids (HCl/HNO3) and the metal content was determined by FAAS/ICP-AES (Table 2). It was observed that quantitative elution was possible with 1.5 N HCl for Ni(II), 1.0 N HNO3 for Cu(II), 1.0 N HCl for Zn(II) and 0.5 N HNO3 for Cd(II) (Table 1).

3.2.4 Sorption capacity and distribution coefficients

The chelating resin (1.0 gm) was equilibrated in the excess of metal ion solution (100 mL, 1000 μg mL-1) by shaking for 1 h at optimum pH conditions to determine total sorption capacity of the resin. Then, the resin was filtered and the concentration of metal ions in the filtrate was determined by FAAS. The amount of metal ions extracted by resin was calculated from the difference in the metal ion concentration before and after sorption (Table 1). The sorption capacity for Ni(II), Cu(II), Zn(II), and Cd(II) was found to be 26000, 49650, 40499 and 52310 μg gm-1, respectively.

Exchange equilibria are often articulated in terms of the distribution coefficient Kd, which is given by the ratio of the equilibrium concentrations of the same metal ion in the resin phase and in the solution.

The distribution coefficient Kd of the metal ions between resin and aqueous phase was determined by batch method. 0.5 gm resin was equilibrated with 100 mL solution containing not more than 130, 248,202, 261μg mL-1 of Ni(II), Cu(II), Zn(II) and Cd(II) for 1 hour at 30°C. The solution was filtered to remove resin and the filtrate was subjected to FAAS / ICP-AES for determination of the metal ion content (Table 1). Kd for Ni(II), Cu(II), Zn(II) and Cd(II) was found to be 4333,7085,6733, 8700, respectively.

3.2.5 Exchange kinetics

To determine rate of loading of metal ions Ni(II), Cu(II), Zn(II) and Cd(II) on resin, the kinetics of sorption was studied by batch method in which 1.0 gm resin was stirred with 100 mL of solution containing 260, 496, 404 and 523 μgmL-1 of Ni(II), Cu(II), Zn(II) and Cd(II) respectively. Aliquot of 1.0 mL of solution was withdrawn at apredetermined interval and analysed using FAAS. The time taken (t1/2) for the 50 % sorption of the metal ions for resin, was found to be 7.0, 4.0, 5.0 and 10.0 (Table 1, Fig. 4) minutes for Ni(II), Cu(II), Zn(II) and Cd(II), respectively, which indicates very good accessibility of (II), Cu(II), Zn(II) and Cd(II) towards chelating sites.

3.2.6 Breakthrough studies

Breakthrough capacity is the actual working capacity of the resin which is more significant and useful than total sorption capacities in ion exchange chromatographic applications. Breakthrough capacity or the effective capacity may be defined as the capacity at the moment when the analyte to be extracted, starts appearing in the effluent. Breakthrough studies were carried out by taking 1.0 gm resin in the column and passing 100 µg mL-1 of metal ions [Ni(II), Cu(II), Zn(II), and Cd(II)] at their optimum pH and flow rates. An aliquot of 20 mL eluant was collected each time and analysed by FAAS/ ICP-AES for the determination of metal ion content (Fig. 5). Breakthrough capacities of resin for Ni(II), Cu(II), Zn(II), and Cd(II), were found to be 7540, 12909, 10529 and 14123µg gm-1, respectively (Table 1).

3.2.7 Stability and reusability of the resin

The reusability of the synthesized resin was examined after several loading and elution cycles. The study was carried out by 0.5 gm of resin beads which were stirred with 100 mL of 300 µg mL-1 solution containing metal ions [Ni(II), Cu(II), Zn(II) and Cd(II)] for 1 hour at room temperature. The elution operations were carried out by shaking the resin with 50 mL of suitable eluant. The results from both tests agreed within 3-4% error for all the metal ions up to 8-10 cycles of sorption and desorption experiments (Fig. 6).

3.2.8 Preconcentration of Ni(II), Cu(II), Zn(II), and Cd(II)

Preconcentration or enrichment step is necessary to bring the sample to the detectable limit of existing detection method when the concentration of metal ions in natural water or effluent water is too low for its direct determination. Therefore, preconcentration factor (PF) or enrichment factor was determined for Ni(II), Cu(II), Zn(II) and Cd(II)on octa-O-methoxy resorcin[4]arene Amberlite XAD-4 polymeric chelating resin.

1000 mL solutions containing 8µg L-1 of each Ni(II), Cu(II), Zn(II) and Cd(II) at pH 3.5, 5.5, 5.0, 8.0 respectively, were passed through the column containing 1.0 gm resin. Metal content in the stripped solution were determined by ICP-AES. The preconcentrating ability of resin was assessed from the elution profile of metal ions by plotting the concentration of effluents as a function of the volume of stripping solutions. 9.5 mL, 1.5 N HCl for Ni(II); 9.2 mL, 1.0 N HNO3 for Cu(II); 10.5 mL, 1.0 N HCl for Zn(II) and 9.0 mL, 0.5 N HNO3 for Cd(II). The preconcentration factors for resin were found to be 105, 95, 108 and 112 for Ni(II), Cu(II), Zn(II) and Cd(II) with 96-98% recovery (Tables 1 and 3, Fig. 7).

3.2.9 Effect of electrolytes

Interference of electrolytes in the spectroscopic determination of metal ions is one of the main problems. To evaluate the selectivity of the synthesized resin, several interfering electrolytes were tested. The limit of tolerance of anions on the sorption of Ni(II), Cu(II), Zn(II) and Cd(II) is defined as that concentration which causes an error of 2-3% in the recovery of these metal ions. The effect of anions and their limit of tolerance on the sorption of Ni(II), Cu(II), Zn(II) and Cd(II) by resin was studied by taking different concentrations of electrolytes. The results are presented in (Table 4). Except Na3PO4 and NaF, all others electrolytes did not interfere between 1.5 - 4.0M concentrations (ranges), which further expand the potential application of resin for the analysis of real samples.

Chromatographic separations

As observed from experimental practice synthesized resin posses very good sorption capacity, distribution coefficient and exchange kinetics, therefore it was further used for the separation of Cu(II), Zn(II) and Cd(II) from their binary and ternary mixtures by column method.

The mixtures of metal ions can be effectively separated by selective adjustment of the pH and eluting agents. Hence, the following mixtures (each 100 μg in 25 mL buffer solution) were passed through the column at the pH of maximum sorption and optimum flow rate. The column effluents were analyzed for the metal ions by spectrophotometry/ FAAS/ICP-AES.

4.1 Separation of a binary mixture:

100 µg each of both Zn (II) and Cd (II) in 25 mL of buffer solution of pH 5.0 were passed through the column at a flow rate of 1.0 mL min1. At this pH, Cd(II) was not sorbed on resin and it came out with the effluent while Zn(II) was retained in the column. Zn (II) was eluted with 24.5 mL, 1.0 N HCl. Quantitative separation was achieved in binary mixture as shown in their separation pattern in Fig. 8(a).

4.2 Separation of a ternary mixture:

100 µg each of Cu (II), Zn (II) and Cd (II) in 25 mL of buffer solution of pH 5.0 were passed through the column at a flow rate of 1.0 mL min1. At this pH, Cd(II) was not sorbed on resin and it came out with the effluent, while Cu(II) and Zn(II) were retained in the column. Cu(II) and Zn(II) were then separated on the basis of selective eluting agents. Zn (II) was eluted first with 25 mL, 1.0 N HCl followed by Cu (II) with 22.5 mL, 1.0 N HNO3. Quantitative separation was achieved in the ternary mixture as shown in their separation patterns in Fig. 8(b).

Limit of quantification

Selectivity and sensitivity are two important factors in the extraction and the separation process. To test the resin's capability to detect trace amounts of metal ions, studies were performed passing 1000 mL of sample solutions containing metal ions in the range of 5-10 μg through the optimized column. The quantification limit for Ni(II), Cu(II), Zn(II) and Cd(II) for resin were found to be 6.4, 5.4, 5.0 and 9.0 μg L-1, respectively, indicating the resin's capability to extract the trace metal ions of interest from the real samples.

Application

To check the applicability of the present method for preconcentrating and determining Ni(II), Cu(II), Zn(II) and Cd(II)), the synthesized resin was subjected to various water samples analyses. For the determination of metal ions by the proposed method, the results are compared by the standard addition technique. In this experiment, 1000 mL of sample volume was spiked with known amount of metal ions and then determined by spectrophotometry/ FAAS/ICP-AES. The data is given in (Table 5).

Comparison with other solid phase extraction methods

Comparison of sorption capacity and preconcentration factor of various adsorbents (Table 6) showed that the synthesized resin has high sorption capacity and good preconcentrating ability for Ni(II), Cu(II), Zn(II) and Cd(II) metal ions.

Conclusion

Octa-O-methoxy resorcin[4]arene Amberlite XAD-4, a newly synthesized polymeric chelating resin was successfully applied for the selective separation, preconcentration and determination of metal ions such as Ni(II), Cu(II), Zn(II) and Cd(II) from real samples. The advantages found for the synthesized resin is its faster exchange rates, better sorption capacity and high preconcentration factors. The resin was found to be highly selective in extracting the analytes in the presence of various electrolytes. The reusability of resin was about 8 to 10 cycles without any significant loss in its sorption behavior. Separations of binary/ternary mixtures of metal ions are possible by control of pH or gradient elution. Sorption capacity of Ni(II), Cu(II), Zn(II) and Cd(II), attained by resin was found to be reasonably better than some already reported solid phase extractants indicating the resin's potential capability to extract the metal ions at trace level in the natural and ground water samples.

Acknowledgment

The authors gratefully acknowledge the financial assistance provided by University Grant Commission (UGC) New Delhi and GUJCOST. The authors also acknowledge CSMCRI (Bhavanagar), GFSU (Gandhinagar), DFS (Ahmedabad), CDRI (Lucknow) for providing instrumental facilities and INFLIBNET, Ahmedabad, for e-journals.