1. INTRODUCTION:

Ion exchange phenomenon has been observed and studied for well over one hundred years. Ion exchange is a widely used technique for selective sorption of ions from different solutions¹. The term 'Ion exchange' describes the process "as water flows through a bed of ion exchange material, undesirable ions are removed and replaced with less objectionable ones". Carriers of exchangeable ions are called "Ion exchangers". Ion exchangers are insoluble materials which carry exchangeable cations or anions. A number of cations can be exchanged with H⁺ due to which the material possess cation exchange properties. A large number of inorganic ion exchangers prepared are amorphous in nature. Many of them have granular nature suitable for column operations. They are known to have a great selectivity towards heavier alkali and alkaline earth metal ions. They have an analytical potential for the recovery and concentration of strongly absorbed trace constituents, which has made their study quite interesting². Heteropoly acid salts of tin(IV) and zirconium are reported in literature to possess better ion exchange characteristics than their single salts^{3, 4}.

Synthetic inorganic ion exchangers are generally produced as gelatinous precipitates by mixing rapidly the elements of groups 3A, 4A, 5A and 6A of the periodic table, usually at room temperature. In recent years, synthetic inorganic ion exchangers are gaining increasing interest due to their specific selectivity and high thermal stability^{5, 9}. Iron(III) antimonosilicate¹⁰ selective for Cd²⁺ and Mo⁶⁺, zirconium succinophosphate¹¹ selective for Th⁴⁺, zirconium phosphomolybdate¹² selective for Bi³⁺, iron(III) tungstophosphate¹³ selective for Hg²⁺ and zirconium iodooxalate¹⁴selective for Ca²⁺ have been studied. Other inorganic ion exchangers are antimony(III) tungstovanadate¹⁵, zirconium antimonophosphate¹⁶, zirconium tungstophosphate¹⁷, tin(IV) selenophosphate¹⁸, antimony arsenophosphate¹⁹, antimony phosphorus-silicon²⁰, stannic phosphotungstate²¹, titanium(IV) tungstophosphate²², antimony(III) silicate²³, antimony tungstate²⁴, zirconium(IV) iodotungstate²⁵, zirconium antimonotungstate²⁶ and zeolite A (SiO₂: Al₂O₃: H₂O: Na₂O)²⁷, tin(IV) tungstosilicate²⁸ , stannic(IV) iodosilicate²⁹, bismuth(III) iodosilicate²⁹, zirconium antimonotungstate³¹ and antimony(III) molybdosilicate³² which are well known ion exchangers.

In our present work the focus was on the synthesis and characterization of an inorganic ion exchanger based on Sb(III). The synthesized exchanger (Antimony(III) tungstophosphate) is a three component ion exchanger which is Pb²⁺selective. The exchanger was characterized on the basis of ion exchange capacity, thermal stability, chemical stability, distribution studies, IR, XRD pattern, and TGA curve studies.

2. EXPERIMENTAL:

2.1 Reagents and chemicals

Antimony trichloride (SbCl₃), Sodium tungstate (Na₃WO₄.2H₂O) and Orthophosphoric acid (H₃PO₄) were Qualigens (India) products. All other reagents and chemicals were also of analytical grade.

2.2 Instrumentation:

pH measurements were performed using a 'Toshniwal Research pH Meter' (Model pH-110) for equilibrium studies. 'Tanco's Electric Rotary Shaker' was used for shaking the solutions for different studies. Thermal studies were carried out by using 'Tanco's Electric Muffle Furnace'. For drying samples, 'NSW India's Oven' was used. 'Samson S-300D Electric Balance' was used for weighing. All glasswares used in this work were of 'Borosil' make. For XRD and FTIR 'Philips Analytical X-Ray B.V. Diffractometer' and 'Thermonicolet IR Spectrophotometer' were used respectively. TGA was obtained with the help of 'Perkin Elmer (Pyris Diamond)' in alumina pan at IIT Roorkee (U.K.)

2.3 Synthesis:

Eleven samples or matrices were precipitated by adding a mixture of 0.1M sodium tungstate and 0.1M orthophosphoric acid solution to 0.1M antimony trichloride solution with continuous stirring in different volume ratios (Table-1). The desired pH was adjusted by adding dil HCl. The precipitates were aged in the mother liquor for twenty four hours at room temperature.

The precipitates of different eleven samples were filtered. After filtration continuous washing of all samples was done to remove excess acid, with distilled water. pH of the effluents were checked with the help of pH paper. When it becomes neutral, then precipitates were assumed to be free from excess acid. Now these eleven different samples were kept in an oven at 40±1⁰C for drying. After about more than 24 hours precipitates become dry and ready for further treatment i.e. granulization. The colour of the precipitates were recorded (Table-1).

For granulization the materials were cracked when they were immersed in to hot water. Granules of eleven samples were, therefore, obtained.

Generation of granules was done by treating them with M HNO₃ solution. For this, granules of eleven samples were kept separately in said acid solutions taken in eleven different conical flasks and 15 minutes continuous shaking was done with the help of electric rotary shaker. The same process was repeated several times with fresh acid solution each time. After settlement of granules, decant the acid and kept the granules in fresh acid solution and leave them as such for twenty four hours at room temperature. After twenty four hours, decant the acid and filter them. Granules are finally washed with distilled water to remove excess acid and dried them in an oven at 40±1⁰C.

Table – 1

Sl. No.	Sample No.	Molar Concentration of Solution			Mixing Volume Ratio			pH Value	*Appearance of Precipitate
Sl. No.	Sample No.	AC	ST	OPA	AC	ST	OPA	pH Value	*Appearance of Precipitate
1	ATP-I	0.1	0.1	0.1	1	1	1	0-1	White powder
2	ATP-II	0.1	0.1	0.1	2	1	1	0-1	Creamy white powder
3	ATP-III	0.1	0.1	0.1	1	2	1	0-1	White powder
4	ATP-IV	0.1	0.1	0.1	1	1	2	0-1	White powder
5	ATP-V	0.1	0.1	0.1	2	2	1	0-1	White powder
6	ATP-VI	0.1	0.1	0.1	2	1	2	0-1	White powder
7	ATP-VII	0.1	0.1	0.1	1	2	2	0-1	White powder
8	ATP-VIII	0.1	0.1	0.1	3	1	1	0-1	White powder
9	ATP-IX	0.1	0.1	0.1	1	3	1	0-1	White powder
10	ATP-X	0.1	0.1	0.1	1	1	3	0-1	Creamy white power
11	ATP-XI	0.1	0.1	0.1	3	3	1	0-1	White powder

ATP - Antimony(III) tungstophosphate ; AC - Antimony trichloride; ST - Sodium tungstate; OPA - Orthophosphoric acid

* The Colour of ppt. is after drying them at 40±1⁰C

Table – 2

S.No.	Sample No.	COBBG	COBAG	I.E.C. (meq/g)
1	ATP-I	White powder	White powder	0.546
2	ATP-II	Creamy white powder	Creamy white powder	0.450
3	ATP-III	White powder	White powder	0.490
4	ATP-IV	White powder	White powder	0.608
5	ATP-V	White powder	White powder	0.480
6	ATP-VI	White powder	White powder	0.542
7	ATP-VII	White powder	White powder	0.630
8	ATP-VIII	White powder	White powder	0.436
9	ATP-IX	White powder	White powder	0.522
10	ATP-X	Creamy white power	Creamy white power	0.601
11	ATP-XI	White powder	White powder	0.406

2.4 Determination of Ion exchange capacity (column method)³³

0.50g (dry mass) of antimony(III) tungstophosphate in H⁺ form was packed in glass column having a glass wool support at the base. M NaNO₃ solution was passed through the column slowly by adjusting the effluent rate at 9-10 drops per minute. The effluent was carefully collected in a 250 ml conical flask. The complete replacement of H⁺ from the ion exchanger by Na⁺ was checked by comparing the pH of the influent (M NaNO₃) and the effluent with the help of pH paper. The collected effluent was titrated against a standard NaOH solution. Then I.E.C. was calculated using a suitable formula (Table-2).

Sample (ATP-VII) was selected for detailed study on account of its highest ion exchange capacity. The sample was synthesized in bulk to fulfill the purpose.

Bulk synthesis was done by mixing 0.1M sodium tungstate, 0.1M orthophosphoric acid solution and 0.1M antimony trichloride solution with continuous stirring in 1:2:2 ratio respectively. The pH was adjusted to 0-1 by adding dil HCl. The precipitate was aged in the mother liquor for twenty four hours at room temperature and filtered. The matrix so obtained was washed to remove the excess acid with distilled water and then it was kept in oven at 40±1⁰C for drying. After drying the matrix was converted in to granules as described earlier. After generation excess acid was removed by washing with distilled water and acid free

granules were dried at 40±1⁰C in an oven. Now these granules are ready for detailed study.

The ion exchange capacity was determined once again by column method using M NaNO₃ solution. The ion exchange capacity was found to be 0.65 which is nearly equal to the previous value. Similarly the I.E.C. of the synthesized material (ATP-VII B) was also determined by the column process for different alkali and alkaline earth metals. Six equal parts of 0.5g each of the exchanger were packed in six different columns and passed some distilled water through

the column to wash and settle the granules to make a bed. Six different metal ion solutions of 0.1 molar concentration were prepared. Each column was treated with a particular salt solution to occur the ion exchange. The flow rate was maintained as one milliliter per minute. The effluents were collected and titrated against a standard sodium hydroxide solution separately to determine the hydrogen ion concentration. The ion exchange capacity was calculated using a suitable formula (Table-3).

Table – 6 Thermal Stability

Sl. No.	Sample No.	Temp (⁰C)	WBH (g)	WAH (g)	LIW (g)	CAH	I.E.C. (meq/g)
1	ATP-VII B	50	0.50	0.50	00	White	0.630
2	ATP-VII B	100	0.50	0.490	0.01	Creamy white	0.630
3	ATP-VII B	200	0.50	0.480	0.02	Light yellow	0.600
4	ATP-VII B	300	0.50	0.470	0.03	Yellow	0.580
5	ATP-VII B	400	0.50	0.470	0.03	Blackish yellow	0.575
6	ATP-VII B	500	0.50	0.460	0.04	Black	0.280
7	ATP-VII B	600	0.50	0.450	0.05	Black	0.267
8	ATP-VII B	700	0.50	0.450	0.05	Black	0.267

WBH- Weight before heating, WAH- Weight after heating, LIW- Loss in weight, CAH – Colour after heating, I.E.C. – Ion exchange capacity

2.7 pH titration method:

This is the another method of determining ion exchange capacity of the exchanger. In this method 0.50g of exchanger was treated with 50ml of the NaCl-NaOH solution. Eleven different samples of NaCl-NaOH solutions were prepared by varying NaCl and NaOH ratios. Eleven equal parts of 0.50g each of the exchanger were treated with eleven different samples of NaCl-NaOH solution separately. Six hours continuous shaking was done with the help of electric rotary shaker and then kept them as such for twenty four hours to maintain the equilibrium. After twenty four hours pH of different samples were measured with the help of pH meter. Please refer table 4 and figure1.

Table-4

Sl. No	Sample No	NaCl –NaOH system		pH Value
Sl. No	Sample No	NaCl solution (ml.)	NaOH solution (ml.)	pH Value
1	ATP-VII B	50	0	2.77
2	ATP-VII B	45	5	6.51
3	ATP-VII B	40	10	6.94
4	ATP-VII B	35	15	7.24
5	ATP-VII B	30	20	8.98
6	ATP-VII B	25	25	9.02
7	ATP-VII B	20	30	9.25
8	ATP-VII B	15	35	9.45
9	ATP-VII B	10	40	9.78
10	ATP-VII B	5	45	10.03
11	ATP-VII B	0	50	10.21

ATP- VII B- Antimony(III) tungstophosphate sample VII synthesized in bulk

2.8 Distribution Studies:

Distribution studies were carried out for ten metal ions by batch process. The metal ion solutions were treated with the fixed amount of the exchanger separately. Then the solutions were shaken for six hours with the help of electric rotary shaker to maintain the equilibrium. After shaking, the solutions were kept as such for twenty four hours at room temperature. After twenty four hours a definite volume of the solution is taken into conical flask with the help of a pipette and titrate against EDTA solution. The metal ion solutions were also titrated against EDTA solution before treatment with the ion exchanger. The distribution coefficient (K_d) values were calculated for metal ions using the following equation.

K_d= I-F A

F W

Where,

I –Burette Reading for the metal ion solutions before treatment with Ion exchanger.

F- Burette Reading for the metal ion solutions after treatment with Ion exchanger

A- Volume of metal ion solution taken

W- Weight of the ion exchanger

Please refer table -7

2.9 IR Study:

FTIR spectrum of the sample was obtained from Instrumentation Centre, IIT Roorkee where Thermonicolet IR Spectrophotometer was available. KBr disc method was used to get the spectrum. The IR absorption spectrum was recorded between 400cm^-1 and 4000cm^-1 (Figure 2).

2.10 XRD:

The X-Ray diffraction pattern of the exchanger was also obtained from Instrumentation Centre, IIT Roorkee where Philips Analytical X-Ray B.V. Diffractometer was available. The diffraction pattern is exhibited in figure 3.

2.11 TGA Curve:

The TGA curve of Antimony(III) tungstophosphate was studied (Figure-4). The curve shows the weight loss from 0⁰C to 800⁰C. There is 4.0% loss at 100⁰C, 6.1% loss at 200⁰C, 6.3% loss at 400⁰C, 6.6% loss at 525⁰C, 6.2% at 650⁰C, 5.1% loss at 700⁰C and 4.8% loss at 759⁰C as shown by the figure 4.

Table –7

K_d values of metal ions on ATP in DMW:

Metal Ion	Taken As	K_d (ml/g)
Mg(II)	Acetate	1.82
Zn(II)	Acetate	3.10
Cu(II)	Acetate	7.22
Mn(II)	Acetate	4.56
Co(II)	Acetate	1.45
Ni(II)	Ammonium Sulphate	29.90
Pb(II)	Nitrate	42.10
Bi(II)	Nitrate	9.18
Cd(II)	Chloride	26.40
Ca(II)	Carbonate	0.94

ATP - Antimony(III) tungstophosphate

2.12 SEM image of Antimony(III) tungstophosphate

Fig-5

2.13 Binary separation:

In the present work binary separations by column method have been done. For separation studies of binary mixture, 0.50g of the exchanger in H⁺from was taken in a glass column (0.6 cm diameter). The column was washed with about 20ml DMW and then a mixture of metal ions was introduced in to the column for exchange. The flow rate of the eluent was maintained at 8-10 drops/minute throughout the elution processes and recycled three times. The column was washed with demineralized water to rinse the sides of the column. The exchanged metal ions were then eluted with appropriate eluents. The flow rate of the effluent was maintained at 1ml/minute throughout the elution process. The effluents were collected and metal ions contents were determined titrimetrically against EDTA titration.

2.14 Removal of hardness causing metal ions :-

For removal studies of hardness causing metal ions, 0.50g of the exchanger in H⁺ form was taken in a glass column. The column was washed with some DMW and then solution of metal ion was introduced in to the column for exchange. The flow rate of the eluent was maintained at 8-10 drops per min throughout the elution process and recycled three times. The exchanged metal ions were then eluted with appropriate eluent. The flow rate of the effluent was maintained at 1ml per minute throughout the elution process. The effluent was collected and metal ion content was determined titrimetrically against EDTA solution.

RESULTS AND DISCUSSION:

Antimony(III) tungstophosphate appears to be a promising ion exchange material. Table 1 and 2 describes the preparation and properties of ion exchange material. Sample ATP-VII was chosen for detailed study owing to its higher ion exchange capacity and stability. The ion exchange capacity was measured for different univalent and bivalent metal ions. The results are summerised in table 5. Studies on the effect of size and charge of the ingoing ion on the ion exchange capacity show that ion exchange takes place in the hydrated form. It is evident from the table that the affinity sequence for alkali metal ions is K⁺> Na⁺> Li⁺ and for alkaline earth metal ions is Ba²⁺> Ca²⁺> Mg²⁺. This sequence is in accordance with the hydrated radii of the exchanging ions. The ions with smaller hydrated radii easily enter the pores of exchanger, resulting in higher adsorption.

The pH titration curve (Figure-1) shows that the ion exchanger releases H⁺ ions easily on addition of NaCl solution to the system in neutral medium, which is indicated by a pH value ~2.77 of the solution. As the volume of NaOH added to the system is increased, more OH^- ions are consumed suggesting in the increase of the rate of ions exchange in basic medium due to the removal of H⁺ ions from the external solution. The I.E.C. calculated from the titration curve found to be 0.67 meq g^-1 of exchanger which is in close agreement to those obtained by the column method.

Studies on the effect of temperature on ion exchange capacity showed that the sample retained some ion exchange capacity for Na⁺ even on heating up to 700⁰C. The ion exchange capacity decreases with increase of temperature. C Janardanan⁽¹⁵⁾ reported similar results while studying the Antimony(III) tungstovanadate.

The material was found to be fairly stable in lower concentration of acids such as HCl, H₂SO₄, HNO₃, CH₃COOH and HCOOH. The ion exchanger completely dissolved in 2M NaOH, 2M KOH and 2M HCl (Table-5).

The absorption band in the infrared spectrum of ATP (Figure-2) is of broad appearance at 3450-3400cm^-1 that shows the presence of hydroxo stretching vibration. The spectrum had a medium intensity band at 1750-1600cm^-1 that shows the presence of deformation vibrations of interstitial water molecule. The spectrum has intensity band at 1023.28cm^-1 and 1382.61cm^-1 that shows the presence of PO₄^3-groups and P=O groups attached to the compound respectively. The spectrum also has a weak intensity band between 800-900cm^-1that shows the presence of tungstate. The spectrum has intense band at 581.80cm^-1that shows the presence of Sb–O group.

The TGA curve (Figure-4) shows a 4% loss of weight at 100⁰C, which may by due to the removal of external water molecule from the exchanger. Beyond 100⁰C the condensation of material must have started, resulting in dehydration due to the removal of the strongly coordinated H₂O molecules from the framework of the exchanger, which continues up to 650⁰C where the weight becomes almost constant. It also involves the production of Sb₂O₃at 700⁰C.

X-Ray Diffraction pattern (Figure-3) of Antimony(III) tungstophosphate shows very weak peaks. XRD and SEM image (Figure-5) indicate that the exchanger is amorphous in form

Table 8 Separation of metal ions achieved on Antimony(III) tungstophosphate Column

Sample

Sepration achieved

Metal ions in µg

% Error

Eluent used

Volume of Eluent used (ml)

Loaded

Recovered

325.00

3048.40

318.50

3017.90

-2.00

-1.00

0.001M HNO₃

0.1M HNO₃

+ 0.5M NH₄OH

1643.00

144.90

1634.78

144.50

-0.50

-0.28

0.001MHNO₃

0.1M HNO₃+0.5 MNH₄OH

4996.00

63.50

4846.12

62.86

-3.00

-1.00

1MHNO₃

0.1MHNO₃

4996.00

325.00

4946.00

318.50

-1.00

-2.00

0.5MHNO₃

+ 0.5MNH₄NO₃

0.05M HNO₃+

0.5MNH₄NO₃

63.50

3048.40

62.23

3018.88

-2.00

-1.00

0.4MNH₄NO₃

0.1MHNO₃

Table 9 Removal of hardness causing metal ions on Antimony (III) tungstophosphate column

Sample	Metal ion	Amount loaded (µg)	Amount found (µg)	Error %	Eluent used
1	Mg²⁺	63.50	62.00	-2.40	0.01MHClO₄
2	Ca²⁺	25.50	25.00	-2.00	1.0M HNO₃

The distribution studies indicates that the material is selective for Pb(II). The decreasing order of K_d values for various bivalent metal ions in distilled water is given as follows:

Pb²⁺> Ni²⁺> Cd²⁺> Bi³⁺> Cu²⁺> Mn²⁺> Zn²⁺> Mg²⁺> Co²⁺> Ca²⁺

On the basis of differences in K_d values, some quantitative separations of analytically important metal ions were performed on the small column of Antimony (III) tungstophosphate.

In binary separations the antimony(III) tungstophosphate separate lead from magnesium and zinc, zinc from cadmium, cobalt from nickel and magnesium from cadmium. Antimony(III) tungstophosphate separates metal ions from 97% to about 100%. Ion exchanger was also applied for water softening. Antimony(III) tungstophosphate removed 98.0% Ca²⁺and 97.6% Mg²⁺from hard water.

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Received on 15.09.2012 Modified on 03.10.2012

Asian J. Research Chem. 5(10): October, 2012; Page 1281-1288

Solution	Wt Before Treatment (g)	Wt After Treatment (g)	I.E.C. (meq/g)
DMW	0.50	0.50	0.63
1M HCl	0.50	0.30	0.24
2M HCl	0.50	Dissolve completely	-
1M HNO₃	0.50	0.34	0.29
2M HNO₃	0.50	0.30	0.12
1M H₂SO₄	0.50	0.30	0.40
2M H₂SO₄	0.50	0.28	0.28
2M CH₃COOH	0.50	0.36	0.25
2M HCOOH	0.50	0.34	0.08
2M NaOH	0.50	Dissolve completely	-
2M KOH	0.50	Dissolvecompletely	-

Cations	Salt of Cation	Solution Concentration	Hydrated Radii (A⁰)	I.E.C. (meq/g)
Na⁺	NaNO₃	0.1M	7.90	0.650
Li⁺	LiCl	0.1M	10.0	0.526
Na⁺	NaCl	0.1M	7.90	0.640
K⁺	KBr	0.1M	5.30	0.680
Mg²⁺	MgCl₂	0.1M	10.80	0.310
Ca²⁺	CaCl₂	0.1M	9.60	0.367
Ba²⁺	BaCl₂	0.1M	8.80	0.408