Abstract

Stannic Phosphotungstate (SnPW) was synthesized by a novel method by the reaction of Tin (II) salt with tungstate and phosphoric acid in the presence of dilute sulphuric acid and hydrogen peroxide. The product was characterized as weakly acidic ion exchanger by pH titrations against alkali metal hydroxides. Ion exchange capacity for alkali and alkaline earth metal ions, distribution coefficients for a number of bivalent metal ions, effect of heat on ion exchange capacity were studied for the ion exchanger. The compound shows excellent chemical stabilities towards acid, neutral and organic solutions. Study of distribution coefficients for various metal ions suggests that SnPW is selective for Pb²⁺, Ba²⁺ and Ca²⁺ in the order, respectively. Structural studies of the compound were carried out using TGA, XRD, FTIR and XRF. Chemical composition of the compound was determined by XRF and the mole ratio was found to be Sn:P:W =  3:1:1.2

The early ion –exchanger were largely inorganic in origin. Subsequently, the synthetic organic resins because of the reproducible preparations dominated the ion-exchange scene with excellent mechanical and chemical stability. In, the recent years, the interest in the inorganic ion-exchangers has been revived with the need for the high temperature separation of ionic components in radio active wastes. In order to accomplish such operations, high selective exchangers are required which are not only stable at high temperature but also have ion exchange properties unaffected by the acidity and high radiation levels. Organic ion –exchange resign are not suitable for such applications, as changes in capacity and selectivity take place on exposure to radiation .Also the degradation take place at the higher temperatures.

Heteropoly acid salts of tin (IV) and zirconium are reported in literature to possess better ion exchange characteristics than their single salts [1-2].  Synthetic inorganic ion exchangers are generally produced as gelatinous precipitates by mixing rapidly the elements of groups 3A, 4, 5A and 6 of the periodic table, usually at room temperature. Sometimes refluxing is also recommended to improve their reproducibility and ion exchange characteristics. The pH is adjusted to 0-1 with the help of an acid or a base. The precipitate thus formed is filtered, washed and dried before putting in demineralized water to obtain granules suitable for column operations. A large number of such materials were prepared by mixing phosphoric, arsenic, molybdic [20], antimonic and vanadic acids with tin, titanium, thorium, zirconium, cerium, iron, antimony, niobium, bismuth, tantalum etc. Alberti [3] and Clearfield [4-5] devoted their studies mainly on the crystalline materials with a view to evaluate their structures based on X-ray studies.

Hybrid inorganic ion exchangers are selective to heavy metals and are useful for removal of arsenic like elements from industrial effluents [21]. However, the analytical studies were initiated and exhaustively made by the research group of Qureshi [6] who mainly concentrated on the amorphous materials. It was based on the observation that in general the gel type materials form good granules or pallets suitable for column operation and thus column chromatographic separations are easily achieved with these materials as compared to the crystalline ones [7-9].

The present work deals with the synthesis of the double salts of tin(IV) phosphotungstate by a novel method. An operationally simple process was developed for the synthesis of this sorbent, which was found compatible with fixed-bed column operation. It also deals with structural characterization, ion exchange studies, thermal and chemical stabilities, determination of distribution coefficients of some metal ions with the compounds and demineralised water.

Experiment

Reagents

Stannous chloride, sodium tungstate, hydrogen peroxide, sulphuric acid and phosphoric acid (Qualigens, INDIA) were used for synthesis. Magnofloc-611 (Allied colloids, UK) was used for coagulation.

Standard solutions for the analytical work were prepared directly by weighing of AR grade reagents or by direct standardization. Demineralized water (DMW) was used for preparation of all the solutions, washing of the compounds and for all practical purposes during the experiments. 

Apparatus

An electric thermostat oven (Microsil, India) was used for drying the samples. Thermo Orion Model 420-A) pH meter was used for pH measurements. REMI centrifuge (R-23) was used for centrifuging of the precipitates. Spectrometric measurements were made by using a UV-VIS Spectrophotometer (DR 4000, Hach-USA). X ray diffractograms were taken from Phillips (ENGLAND), X-ray diffractometer using Kα_1,2 radiations. Infrared studies were done with a Perkin Elmer Fourier Transform Infrared Radiation instrument. Electric furnace (Microsil, India) was used for heating the samples at elevated temperatures. Thermo Gravimetric Analysis was carried out using the Mettler-Tolledo Star system. Elemental analysis of the compounds was carried out with the help of X-Ray Fluorescence. All glassware including measuring pipettes, burettes and flasks and cylinders used were of borosilicate glass with A-class certificate. 

Synthesis of Tin (IV) Phosphotungstate

Sodium tungstate solution (1M) was added to stannous chloride solution (1N) in the presence of  sulphuric acid (5N), Hydrogen peroxide (0.3%) and  Phosphoric acid (5N). Flocculating agent (1.5ppm) was added to the reaction mixture. The whole process of synthesis is carried out at pH 0 -1. Reaction time of 1 hr was allowed for complete formation of gel and then the gel precipitates were washed with demineralised water till acid free. The resulting precipitates were left in water at room temperature for 24 hours for growth of the particle size. Supernatant liquid was decanted and washed the precipitates with DM water. Then the precipitate was centrifuged at 900 R.P.M. for 30 minutes and dried in hot air circulated oven at 45^oC. The whole mass of precipitates became compact solid after about 72 hours of drying.  The dried product broke down to small granules when immersed in water. The granular particles of the compound were again air dried at 45^oC in oven and collected the material. The product was found compatible for fixed bed column operations. Five such samples were synthesized at different intervals of time for various studies.

Characterisation of the Material as Ion Exchanger
 

Ion Exchange Capacity 

Ion exchange capacity of the synthesized compound sample was determined by column operation .The ion-exchanger in H⁺-form was placed in a glass column containing a cotton support. Different salt solutions (0.1 M) were used as eluants. 200mL of each electrolyte was passed through the ion-exchanger column made from 0.5 g of the exchanger. Hydrogen ions eluted from the column were determined titrimetrically. Ion-exchange capacity of the material was determined for some uni and bivalent cations.  The results are given in Table 1. 

Regeneration of Ion-Exchangers 

The exhausted exchanger was regenerated by keeping it overnight in hydrochloric acid (1 N). It was then washed with demineralised water to remove excess acid. Then the ion exchange capacity was determined for Na⁺ ions. This procedure was repeated several times. The observation after each time has been given in Table 2. it was observed that even after five regenerations, the ion-exchanger loses only about 5% of its original capacity. 

Chemical stability 

0.5 g of each of the exchanger compound was kept separately in 20 mL of different solvents, viz, DM H₂O, 0.1M solutions of NaCl, NaNO₃, NaOH, KOH, HCl, and HNO₃ , Acetone, and Ethanol for 24 hours and then filtered. Tin [10], Phosphorus [11] and Tungsten [12] in the filtrate were determined spectrophotometrically.

Thermal Stability 

The ion-exchanger sample was dried at different temperatures from 100^o C to 600^o C in a muffle furnace. For each 100^oC rise in temperature, loss in weight% and loss in ion exchange capacity for sodium ions as a function of temperature were determined for the compound. The values of observations are given in Table 4 and Table 5 respectively. Thermogravimetric Analysis of the compound was also carried out to find the loss of weight with increasing temperature up to 900^oC.  

pH titrations   

The pH titrations with LiOH, NaOH and KOH were performed by the added salt method.  0.2 g of the exchanger was kept in contact with varying volumes of the base (0.1 N) and the sodium chloride solution (0.1N) for 24 hours with intermittent shaking of the bottles. pH of the supernatant liquid was measured. 

Sorption Studies 

Distribution coefficients (K_d) for some of the metal ions were determined in DMW. The K_d values were calculated by using the formula:             

        K_d = (I-F) x 20 /0.2 x F   mL/g    [19] 

I = Volume of 0.01 M EDTA consumed by the cations before equilibrium.
F = Volume of 0.01 M EDTA consumed by the cations after equilibrium.
All the cations used were determined by titrating against 0.01M EDTA. Distribution coefficients of those ions with demineralised water are given in Table 6. 

Structural studies 

Elemental analysis of the compound was carried out using X-Ray Fluorescence method.  The results of analysis are given in Table 7. Other structural studies like X-ray diffractograms (powder method), IR spectra and thermal analysis were carried out for the material. X-Ray diffractogram has been given as Figure 1 and infra red absorption bands with their corresponding functional groups of the compound are given in Table8. 

Results and Discussion 

Three-heteropoly acid salts of tin were synthesized. These were characterized as weakly acidic ion-exchangers. Extensive studies of these ion-exchangers were carried out. They are discussed as below: 

Synthesis of stannic phosphotungstate 

Stannous chloride reacts with sulphuric acid in the presence of hydrogen peroxide to form an oxy-acid salt of Sn⁴⁺. Oxy-acid salt of Sn⁴⁺ further reacts with excess hydrogen peroxide to form peroxyhydrates [13]. This peroxyhydrate remains as solution of its component[13] and reacts with sodium tungstate in the presence of phosphoric acid to form gel like precipitates. Thus hydrogen peroxide helps in controlling the process by slowing down the reaction [14]. The method was repeatedly used to synthesise the compound and ion exchange capacity of the samples from each lot produced was determined. The values obtained, as given in Table 3, shows a reproducible ion-exchange capacity within experimental errors. 

Ion exchange capacity 

Clearfield observed that Ion-exchange capacity of various metal ions of the compounds increase with decrease in the size of the entering ion [15]. Nabi et al observed that besides the size of the ion, the effective charge on the cation [16] also plays role in ion exchange capacity of the exchangers, i.e., IEC decreases with increasing charge of the ion. Hydrated ionic radii are in the reverse order of their bare ionic sizes for a series of metal ions in a group.  From the values of the experiments of ion exchange capacity (Table-1), it is observed in the present work that the compound exhibits both of the above-mentioned theories. 

Based on characterization by TGA and XRF analysis, an empirical formula on the basis of Alberti equation is proposed below: (SnO₂)₄ (H₃PO4)₅ (H₂WO₄) 8 H₂O. It has been assumed that water of crystallisation is removed when the material is heated up to 200^oC. The loss in weight at 200^oC has been used to calculate the number of water of crystallisation molecules, which after rounding off comes out to be 8. 

Regeneration of Ion–exchanger 

Each ion exchanger after exhaustion was regenerated with hydrochloric acid. The H⁺ ions of HCl replace cations thus exchanged with the replaceable hydrogen ions of the exchangers. It was confirmed by repeated regenerations of each compound and its use for ion exchange with sodium ions. The ion-exchangers exhibit repeated ion exchange capacity with a maximum loss of about 5 % of their original capacities (Table-2).

Table  2: Ion Exchange capacity of SnPW for different regeneration cycles


Reproducibility 

The above ion exchanger was synthesized repeatedly using the same method of synthesis. Every time, it was observed that the ion exchanger produced so was having similar properties of ion exchange capacity (Table-3) within the limits of experimental errors. This proves the authenticity of the method of synthesis. 

Table  3: Ion Exchange capacity of Sn-P-W to show reproducibility.

Chemical stability   

The ion-exchanger shows excellent chemical stability towards strong acids, salt solutions, alkali solutions and organic solvents. This characteristic shows that the ion-exchangers can be safely used in organic and most of the aqueous media without dissolution problem. Tin [10], Phosphorus [11] and Tungsten [12] in the filtrate were determined spectrophotometrically and found not traceable”. 

Thermal stability 

The compound is quite stable thermally. Weight loss of the ion-exchanger heated at various temperatures was recorded (Table-4). There is a sharp decline in weight of the compounds after 200^o C probably due to the loss of coordinated water molecules, which is supported by a sharp decline in its ion-exchangers capacity (@ 40%) at 300^oC. It also supports the hypothesis that most of the ion-exchangers capacity is due to the coordinated water molecules. A gradual loss in weight is observed for the exchanger on heating beyond 300^o C, which may be due the reason that hydroxyl group, combines to form water molecules. Ion -exchange capacity was determined for all the samples heated at various temperatures from 100^o C to 600^o C. The ion-exchange capacity decreases with increasing temperature up to 300^o C and on successive heating.

Table  4: Weight loss of Sn-P-W at different temperatures


Table  5: Ion Exchange capacity of Sn-P-W at different temperatures

Sorption studies  

Distribution coefficients for a number of metal ions were determined on the ion-exchanger.  SnPW shows better selectivity for Pb²⁺ and Ba²⁺  ions. The greater selectivity for the ions of respective compounds may be due to the reason that the sizes of these cations just match the size of the cavity in the respective exchanger matrix. These cations form stronger metal-oxygen bonds and are hence preferred over the cations with inappropriate sizes, which form weaker bonds with framework oxygen.  

Elemental analysis by XRF 

XRF studies indicate that SnPW ion -exchanger is composed of Sn, P and W in the mole ratio of 3:1:1.2.
(Table 7). 

X-Ray Diffractogram of Sn-P-W 

X-Ray diffractogram of the compound shows no sharp peaks (Figure 1). The compound seems to be amorphous in nature. 

 

Assignment of IR absorption bands of SnPW [17-18] 

The absorption band in the infrared spectrum of SnPW (Table 8) is of medium and weak intensities at 470 cm-1, 490 cm-1 and 506 cm-1 respectively and shows the presence of Sn-O group. High intensity band at 1050 cm^-1 indicate the compound has PO₄^3- groups. The spectrum has a medium intensity band at 1398 cm^-1 that shows the presence of P=O groups attached to the compound.  The spectrum shows that coordinated water molecules are present with the compound as indicated by a medium intensity band at 1636cm^-1. Medium intensity band at 2360cm^-1 and a weak intensity band at 3389cm^-1 indicate the presence of O-H groups.

Table  8: Assignment of the Infra Red Absorption bands of SnPW

Conclusions

The Ion exchanger, SnPW, was synthesized using novel method of synthesis. The method allows the reaction time, which was instant in most of the earlier synthesis. Stannous chloride is used instead of stannic chloride as the later is corrosive and needs specific attention in handling. Sulphuric acid was used for pH adjustment. Hydrogen peroxide is used for oxidation of tin(II) to tin(IV) ions allowing the acid salts to remain in solution for reaction. Dilute polyelectrolyte solution is added for coagulation of the precipitates. The compound is amorphous as envisaged by the absence of sharp peaks in X-Ray Diffractogram and was used in column operations for various applications. It is specifically selective to Lead.

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