Langmuir 2001, 17, 2589-2593
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Polystyrene Thorium(IV) Phosphate as a New Crystalline and Cadmium Selective Fibrous Ion Exchanger. Synthesis Characterization and Analytical Applications K. G. Varshney* and Namrta Tayal Analytical Research Laboratory, Department of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh 202 002, India Received April 12, 2000. In Final Form: January 31, 2001 Some investigations on the preparative procedures of fibrous polystyrene thorium(IV) phosphate (PStThP) are reported. This new fibrous ion exchanger has been characterized on the basis of scanning electron microscopy, X-ray diffraction analysis, IR, thermogravimetric analysis, and ion exchange studies. The ion exchange capacity for Na+ ion is found to be 4.52 mequiv/dry g basis. Separation factors and Kd values for various cations at different concentrations have been determined, and a marked selectivity for Cd(II) has been found. The unusual selectivity for Cd(II) should lead to some useful practical applications.
Introduction Fibrous ion exchangers exhibit a high efficiency in the process of sorption from gaseous and liquid media,1 and hence they have drawn the attention of researchers, particularly environmentalists. These materials consist of monofilaments of uniform size ranging in diameter between 20 and 300 µm. Their main advantage is that they can be produced in various forms, such as staples, cloth, and nonwoven materials, opening up many possibilities for new technological processes. Although granular ion exchangers ensure very favorable parameters for ion exchange processes, their application in large scale processes is hardly possible because of the high resistance of filtering layers. This difficulty is eliminated when fibrous ion exchange materials are used since the layer resistance is easily predetermined by the density of a fibrous material packing in accordance with technological requirements. It is for this reason that the ion exchange fibers are of great importance.2,3 Fibrous inorganic ion exchangers are very interesting because they can be used to prepare inorganic ion exchange papers suitable for chromatographic cation separations or inorganic membranes without a binder.4,5 Recently, some efforts have been made in our laboratories to synthesize new hybrid types of fibrous ion exchange materials, i.e., obtained by the combination of organic polymeric species and inorganic ionogenic groups. This property has been illustrated very well during our studies on an acrylonitrile-based cerium(IV) phosphate (ANCeP)6 pointing to the fact that these materials can be of great importance in industrial and environmental applications because of the possibility of preparing them into different pratically useful forms. Also the main use of these materials is in the atomic power plants for the purification * Author for correspondence. (1) Soldatov, V. S.; Shunkevich, A. A.; Sergeev, G. I. React. Polym. 1988, 7, 159. (2) Kotze, M. H.; Cloete, F. L. D. Ion Exch. Adv., Proc. IEX 92, 1992, 89. (3) Bajaj, P.; Goyal, M.; Chavan, R. B. J. Appl. Polym. Sci. 1994, 51, 423. (4) Alberti, G.; Massucci, M. A.; Torracca, E. J. Chromatogr. 1967, 30, 579. (5) De, A. K.; Chowdhury, K. Sep. Sci. 1975, 10, 39. (6) Varshney, K. G.; Tayal, Namrata; Gupta, U. Colloids Surf., A 1998, 145, 71.
of water at high temperature and high pressure conditions. So, a crystalline polystyrene thorium(IV) phosphate has been prepared which possesses a fibrous structure with high mechanical strength and an exceedingly high ion exchange capacity (4.52 mequiv/dry g). The present paper summarizes the synthesis, ion exchange behavior, and analytical applications of this new material. Thorium(IV) phosphate has been prepared earlier as a fibrous inorganic ion exchanger.7 Experimental Section Reagents and Chemicals. Thorium nitrate (Th(NO3)4‚5H2O) and styrene (C6H5CHCH2) were the CDH (India) products having 99% and 93% purity, respectively, while orthophosphoric acid (H3PO4) was a Qualigens (India) product having 98% purity. All other reagents and chemicals were of Analytical Reagent grade. Preparation of the Reagent Solutions. Solutions of thorium nitrate were prepared in 1 M HNO3 while those of styrene were prepared in ethanol. The solutions (2 M) of orthophosphoric acid were prepared in demineralized water. Synthesis of Polystyrene Thorium(IV) Phosphate (PStThP). A number of samples were prepared by adding one volume of 0.1 M Th(NO3)4‚5H2O solution in two volumes of a (1:1) mixture of 2 M H3PO4 and styrene (8.69-868.94 mmol) dropwise with constant stirring using a magnetic stirrer at a temperature of 90 ( 5 °C. The resulting slurry was stirred for 5 h at this temperature, filtered, and washed with demineralized water (pH ∼ 4). On drying at 5-10 °C the precipitate resulted into a sheet which was crushed into small pieces and converted into the H+ form by treating with 1 M HNO3 for 24 h with occasional shaking, and intermittently replacing the supernatant liquid with fresh acid. However the product dried at room temperature was not stable and was found to melt on standing, i.e., the material not obtained at room temperature. The material obtained was then washed with demineralized water to remove the excess acid before drying finally at 45 °C and sieved to obtain particles of 50-70 mesh size. Table 1 summarizes the synthesis of various samples of the material. Since the sample PStThP-5 shows the maximum ion-exchange capacity, it was selected for further studies. Ion-Exchange Capacity. The ion-exchange capacity of the sample was determined8 leading a column containing 1 g of the material (H+-form) in a glass tube of internal diameter ∼1 cm, fitted with glass wool at its bottom. A 250-mL portion of 1 M NaNO3 solution was used as eluant, maintaining a very slow (7) Alberti, G.; Costantino, U. J. Chromatogr. 1970, 50, 482. (8) Varshney, K. G.; Agrawal, K.; Agrawal, S.; Saxena, V.; Khan, A. R. Colloids Surf. 1988, 29, 175.
10.1021/la000552v CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001
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Table 1. Synthesis of Various Samples of Polystyrene Thorium (IV) Phosphate sample no.
mmol of styrene/1000 mL
Na+-ion exchange capacity (mequiv/dry g)
PStThP-1 PStThP-2 PStThP-3 PStThP-4 PStThP-5 PStThP-6 PStThP-7 PStThP-8 PStThP-9 PStThP-10 PStThP-11 PStThP-12 PStThP-13
8.69 17.38 43.45 86.89 173.79 260.68 347.57 434.47 521.36 608.26 695.15 782.04 868.94
3.00 3.15 3.35 4.05 4.52 4.20 4.05 3.82 3.55 2.40 1.96 1.65 1.28
flow rate (∼0.5 mL min-1). The effluent was titrated against a standard alkali solution to determine the total H+ ions released. Elution Behavior. The extent of elution was found to depend on the concentration of the eluant. Hence a fixed volume (250 mL) of the NaNO3 solution of varying concentrations was passed through the column containing 1 g of the exchanger and the effluent was titrated against a standard alkali solution for the H+ ions eluted out. The optimum concentration of the eluant for a complete elution of H+ ions in 250 mL of NaNO3 solution was found to be 1 M. A similar column containing 1 g of exchanger was then eluted with a NaNO3 solution of 1 M concentration in different 10 mL fractions with a minimum flow rate as described above. This experiment was conducted to find out the minimum volume necessary for a complete elution of H+ ions, which reflects the efficiency of the column. Thermal Studies. One-gram samples of the material were heated at various temperatures for 1 h each in a muffle furnace, and their ion-exchange capacity was determined by the column process after cooling to room temperature. The thermogravimetric analysis/differential thermogravimetry (TGA/DTG) analysis was carried out using a Cahn Thermobalance, model 2050. The value of external water molecules “n” can be calculated using Alberti’s equation9
18n )
X(M + 18n) 100
X is the percent weight loss and (M + 18n) is the molecular weight of the exchanger where M is [(ThO2)(H3PO4)3(C6H5CHCH2)2]‚nH2O having molecular weight 766. Composition. The sample was dissolved in 2 M H2SO4 solution. Thorium(IV) was determined by an atomic absorption spectrophotometer, Shimadzu model AA-640, while phosphate was determined spectrophotometrically by the phosphovanadomolybdate method10 using a UV-vis spectrophotometer, Elico model SL151, as follows: To the 10-mL sample solution, taken in a 100-mL volumetric flask, were added 50 mL of demineralized water (DMW), 10 mL of ammonium vanadate solution (1.25 g of ammonium vanadate dissolved in 250 mL of DMW + 20 mL of concentrated HNO3, diluted to 500 mL), and 10 mL of ammonium molybdate solution (12.5 g of ammonium molybdate dissolved in 250 mL of DMW). It was then diluted up to the mark before taking its absorbance at 460 nm against a reagent blank prepared in the same manner. Elemental Analysis. Elemental analysis was carried out using a Carlo-Erba 1106 elemental analyzer. Chemical Stability. Different 250-mg portions of the material were kept in 25 mL of the various mineral acids, bases, and salt solutions of different concentrations for 24 h each, with intermittent shaking. The supernatant liquid was analyzed for the thorium(IV) and phosphate contents by the methods mentioned above. (9) Alberti, G.; Torracca, E.; Conte, A. J. lnorg. Nucl. Chem. 1996, 28, 607. (10) Vogel, A. I. Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman: New York, 1978; p 756.
pH Titrations. pH titrations were performed by the Topp and Pepper’s method11 using an Elico pH meter, model LI-10. Various 500-mg portions of the exchanger in H+ form were placed in each of the several 250-mL conical flasks containing equimolar solutions of 0.1 M alkali metal chlorides and their hydroxides in different volume ratios to maintain the ionic strength constant, i.e., 0.2. The total volume was kept at 50 mL. The pH of the solution was recorded after keeping the mixture at room temperature for 6 days to attain equilibrium. It was plotted against the milliequivalents of OH- ions added. Infrared Spectroscopic Studies. The IR studies were carried out by the KBr disk method using a Nicolet 5 DX. Scanning Electron Microscopic Studies. Scanning electron microscopy (SEM), studies were done with a JEOL model JSM 840. X-ray Diffraction Studies. X-ray diffraction studies were made on a Philips X-ray diffractometer, model PW 1710. Distribution Studies. A 200-mg portion of the exchanger in H+ form was kept in 20 mL of the solvent for 24 h, with intermittent shaking to attain equilibrium. The initial metal ion concentration was so adjusted that it did not exceed 3% of the total ion-exchange capacity of the material. The pH values of the pure solutions were kept at 0-2, while that of the metal ion solutions were maintained at 6-7. The metal ions in the solution before and after equilibrium were determined by EDTA titration12 and the distribution coefficients, Kd were calculated by the formula
Kd )
I-F V (mL g-1) F M
where I and F are the initial and final amounts of the metal ion in the solution phase, V is the volume (mL) of the solution, and M is the amount (g) of the exchanger. Separations Achieved. Several binary separations were tried using a column of ∼0.6 cm i.d. containing 2 g of the material. The column was washed thoroughly with demineralized water, and the 4 mL of the mixture to be separated was loaded on it, maintaining a flow rate of ∼2-3 drops min-1 (0.15 mL min-1). The mixtures having a metal ion concentration of 0.01 M are as follows: Ba(NO3)2-Cd(NO3)2, Hg(NO3)2-Cd(NO3)2, and Zn(NO3)2-Cd(NO3)2. The separation was achieved by passing a suitable solvent through the column as eluant. The metal ions in the effluent were determined quantitatively by EDTA titrations.
Results and Discussion This study highlights certain interesting features of the polystyrene thorium(IV) phosphate. The material shows an exceedingly high ion-exchange capacity for Na+ ions (4.52 mequiv/dry g), which is much higher than the ion-exchange capacity shown by the other ion exchange materials reported so far in these laboratories.13-15 Table 2 shows the ion-exchange capacity of the material for various metal ions. Further, the material was obtained in the form of a sheet. The SEM of the material shows its fibrous structure (Figure 1). It appears to be stable chemically as is evident from Table 3. Inorganic ion exchange papers of PStThP can be easily prepared, and these papers compare favorably with those of cerium-based materials6 with regard to their stability with strong reducing agents. This property is important in chromatographic separations where reducing agents are often used as eluants or spot-test reagents. (11) Topp, N. E.; Pepper, K. W. J. Chem. Soc. 1949, 3299. (12) Reilly, C. N.; Schmidt, R. W.; Sadek, F. S. J. Chem. Educ. 1959, 36, 555. (13) Niwas, R.; Khan, A. A.; Varshney, K. G. Colloids Surf. A 1999, 150, 7. (14) Varshney, K. G.; Pandith, A. H.; Gupta, U. Langmuir 1998, 14, 26, 7353. (15) Niwas, R.; Khan, A. A.; Varshney, K. G. Indian J. Chem. 1998, 37A, 469.
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Langmuir, Vol. 17, No. 9, 2001 2591 Table 4. Thermal Stability of Polystyrene Thorium(IV) Phosphate after Heating to Various Temperatures for 1 h drying temp Na+ ion exchange (°C) capacity (mequiv/dry g) 45 100 200 400 600 800 a
4.52 4.46 3.26 2.58 1.45 0.18
change in color
% retention of ieca
white white dirty white light gray light gray dark gray
100.00 99.12 72.45 57.10 32.10 3.98
iec, ion-exchange capacity.
Figure 1. The electron micrograph of polystyrene thorium(IV) phosphate. Table 2. Ion-Exchange Capacity of Polystyrene Thorium(IV) Phosphate for Various Metal Solutions metal solution
ion-exchange capacity (mequiv/dry g)
LiCl NaNO3 KCl Mg(NO3)2 Ca(NO3)2 Sr(NO3)2 BaCl2
1.55 4.52 2.43 2.95 3.75 4.29 4.86
Figure 2. Concentration plot of polystyrene thorium(IV) phosphate having 1 g/250 mL solid/solution ratio.
Table 3. Chemical Stability of Polystyrene Thorium(IV) Phosphate in Various Acid, Alkali, and Salt Solutions amount dissolved (mg)
a
solvent (25 mL)
thorium
phosphate
1 M HCl 2 M HCl 4 M HCl 1 M HNO3 2 M HNO3 4 M HNO3 1 M H2SO4 2 M H2SO4 4 M H2SO4 2 M NaNO3 2 M KNO3 0.05 M NaOH 0.1 M NaOH 0.1 M KOH 0.1 M NH4OH 0.5 M NH4OH
0.59 0.76 1.10 0.85 1.10 1.56 1.36 a
0.04 0.06 0.07 0.05 0.10 0.16 0.14 a
0.06 0.12 1.12 0.72 1.40 1.38 1.42
0.00 0.06 0.02 0.05 0.55 0.16 0.25
Dissolved completely.
The drying temperature was found to be an important feature in the synthesis of PStThP. The material was obtained in the form of ion exchange paper when dried at 5 ( 10 °C (keeping in the refrigerator). It was stable even up to 600 °C. The product dried at room temperature, however, was not stable and was found to melt on standing. It is a unique feature of this material. Thermally, the material appears to be highly stable. It retains approximately 57% of its ion-exchange capacity when heated to 400 °C but retains only 32.10% of its ionexchange capacity when heated to 600 °C as shown in Table 4. This is important because the main use of these materials is in atomic power plants for the purification of water at higher temperature and high-pressure conditions.
Figure 3. Equilibrium pH titration curves for polystyrene thorium(IV) phosphate with various alkali metal hydroxides: b, NaOH/NaCI; 4, KOH/KCI; O, LiOH/LiCI.
The elution behavior indicates that the exchange is quite fast and almost all the H+ ions (exchange sites having PO4) are eluted out in the first 160 mL of the effluent from a column of 1.0 g of exchanger. The optimum concentration of the eluant was found to be 1 M (Figure 2) for a complete elution of H+ ions from the above column. Moreover the exchange takes place in one step as indicated by the pH titration curves obtained under
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Figure 4. TGA curve of polystyrene thorium(IV) phosphate. Table 5. Composition of Polystyrene Thorium(IV) Phosphate wt of the material (mg) 50
mmol of the components Th(IV) PO43C H 0.324
1.07
0.69
1.32
mole ratio Th(IV):PO4:St 1:3:2
equilibrium conditions (Figure 3) for LiOH/LiCl, NaOH/ NaCI, and KOH/KCI exchanges, which may be due to the fact that the first ionization of phosphoric acid is much faster when compared to the other two (pKa values of H3PO4 being 2.12, 7.21, and 12.30). In this regard these materials are similar to the heavy metal arsenates prepared earlier as ion exchangers.16,17 The three pKa values for arsenic acid are 2.25, 6.77, and 11.60, respectively. A fast release of the H+ ions in aqueous media is also supported by the low pH (∼2) of the salt solution with which the material is kept in contact for some time. It shows a strong cation exchange behavior of the material. The adsorption behavior for alkali metals was observed to be in the order K(I) > Li(I) > Na(l) in acidic media. It is the reverse of the order Li(I) > Na(I) > K(I) obtained in the basic media. On the basis of its chemical analysis (Table 5), the molar composition of thorium, phosphate, and styrene was found to be 1:3:2 suggesting the formula [(ThO2)(H3PO4)3(C6H5CHCH2)2]‚nH2O. Styrene may undergo polymerization thus enhancing the polymeric behavior of polystyrene thorium(IV) phosphate. However, a crystal structure cannot be given at this stage, which is a separate study under progress. The thermogram (Figure 4) shows a 8% weight loss up to 230 °C. It may be due to the removal of external water molecules “n” from the exchanger. The value of “n” was found to be ∼3.7 using Alberti’s equation. On heating beyond 230 °C the condensation process starts as is indicated by a further weight loss up to around 360 °C. Beyond this temperature a more horizontal portion of the curve starts18 which continues up to 550 °C. The horizontal portion after 550 °C indicates the formation of pyrophosphate.18
Figure 5. IR spectrum of polystyrene thoriurn(IV) phosphate.
The IR spectrum (Figure 5) confirms the presence of external water molecules in addition to the metal oxides and metal hydroxides (at the PO4 sites) present internally in the material. The metal oxide and metal hydroxide (16) Qureshi, M.; Kumar, R.; Rathore, H. S. J. Chem. Soc. A 1970, 272. (17) Qureshi, M.; Nabi, S. A. J. lnorg. Nucl. Chem. 1970, 32, 2059. (18) Duval, C. Inorganic Thermogravimetric Analysis; Elsevier: Amsterdam, 1953; pp 492 and 497.
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Langmuir, Vol. 17, No. 9, 2001 2593
Table 6. Kd Values of Metal Ions on Polystyrene Thorium(IV) Phosphate in DMW, Perchloric Acid, Nitric Acid, and Hydrochloric Acid Media Having the Solid Solution Ratio as 200 mg/20 mL HClO4
HNO3
HCl
metal ions
DMW/H+
0.01 M
0.1 M
1M
0.01 M
0.1 M
1M
0.01 M
0.1 M
1M
Mg(II) Ca(II) Sr(II) Ba(II) Pb(II) Mn(II) Cd(II) Cu(II) Fe(III) Al(III) Co(II) Hg(II) Ni(II) Zn(II)
900.00 550.00 836.00 1125.00 342.00 655.00 1900.0 900.00 900.00 1200.00 836.00 525.00 1125.00 350.00
325.00 288.00 224.00 125.00 633.00 418.00 1900.0 825.00 350.00 1200.00 655.00 525.00 572.00 350.00
220.00 125.00 100.00 80.00 224.00 300.00 1900.0 820.00 300.00 1066.00 424.00 28.00 436.00 350.00
112.00 78.00 64.00 42.00 175.00 149.00 1900.0 350.00 225.00 956.00 220.00 10.00 390.00 125.00
250.00 250.00 224.00 150.00 615.00 200.00 1900.0 500.00 255.00 1200.0 650.00 175.00 572.00 350.00
178.80 70.00 80.00 80.00 143.00 170.00 1900.0 314.00 236.00 950.00 572.00 39.00 550.00 125.00
112.00 36.66 64.00 30.00 92.00 125.25 1900.0 90.00 150.00 836.00 218.00 26.00 400.00 120.00
325.00 250.00 300.00 125.00 525.00 200.00 1900.0 616.00 350.00 1250.0 424.00 90.00 300.00 300.00
220.00 125.00 125.00 100.00 214.00 150.00 1500.0 428.00 300.00 1153.0 300.00 56.00 172.00 78.00
78.00 78.00 75.00 66.66 100.00 100.00 1225.0 212.00 150.00 616.00 272.00 38.00 125.00 49.00
Table 7. Binary Separations of Metal Ions Achieved on Polystyrene Thorium(IV) Phosphate Columna no.
separation achieved M1-M2
1
Hg(II)-Cd(II)
2
Zn(II)-Cd(II)
3
Ba(II)-Cd(II)
a
amt loaded (µg) M1 M2 2900.8 980.55 2403.3
amt found (µg) M1 M2
error (%) M1 M2
1910.8
2800.0
1910.8
-3.45
0.00
1910.8
1013.2
1882.7
3.33
-1.47
1910.8
2368.9
1910.8
-1.43
0.00
eluant used Hg: Cd: Zn: Cd: Ba: Cd:
0.1 M HClO4 1 M HNO3 0.1 M HCl 1 M HNO3 0.01 M HClO4 1 M HNO3
vol. of eluant (mL) 40 50 50 50 60 50
Where M1 is Hg(II), Zn(II), and Ba(II) and M2 is Cd(II).
Figure 6. X-ray diffraction pattern of polystyrene thorium(IV) phosphate.
bands are observed at 628 cm-1, while bands at 546 and 1083 cm-1 indicate the presence of a phosphate group.19 The presence of external water molecules is indicated by the band at 1631 cm-1, in addition to its usual range at 3436 cm-1.19 The absorption band at 2928 cm-1 confirms the presence of polystyrene in the structure, while the one at 1402 cm-1 is highly characteristic of the aromatic ring in the structure.19 The X-ray diffraction pattern of the material (Figure 6) as compared to thorium phosphate itself is not very sharp. The counts are very low and with broad and noisy peaks, indicating its poorly crystalline behavior. The distribution studies (Table 6) point out that PStThP is selective for Cd(II) ions. Hence the utility of the material was demonstrated by achieving separations of great analytical significance; for example, on its columns the possible separations are Cd(II)-Zn(II) and Cd(II)-Hg(II). (19) Rao, C. N. R. Chemical Applications of Infrared Spectroscopy; Academic Press: New York, 1963; pp 353, 356, 48, and 159.
Figure 7. Separation of Hg(II)-Cd(II), Zn(II)-Cd(II), and Ba(II)-Cd(II) on polystyrene thorium(IV) phosphate columns: (a) 0.1 M HClO4; (b, d, f) 1 M HNO3; (c) 0.1 M HCl; (e) 0.01 M HClO4. Where a, b, c, d, e, and f are the possible eluants for the separation of metal ions.
Also a binary separation, Cd(II)-Ba(II) was achieved on its column (Figure 7). As the material is selective for Cd(II), the elution of this metal ion by 0.1 M HClO4 and 0.1 M HCl is the least. However, the separation peaks of Cd(II) slightly overlap as is evident from their elution behavior. The separations are quite precise as the results in Table 7 show. Acknowledgment. The authors thank the C.S.I.R., India for the financial assistance. LA000552V
