Indi an Journal of Chemistry Vo l. 42A, March 2003, pp. 559-563

Synthesis, characterization and ion exchange behaviour of a new phase of tin tungstate

Alpana H Parikh & Uma V Chudasama* Applied Chemistry Department, Faculty of Technology and

Engineering, M.S.University of Baroda, Vadodara 390 DOl, India

Received 19 March 2002; revised 21 October 2002

A new phase of tin tungstate, an inorganic ion exchanger of the class of tetravalent metal acid (tma) salt has been synthesized with good ion exchange properties. The material has been character­ized for elemental analysis, thermal analysis (TGA, DSC). FfIR spectroscopy , X-ray analysi s and surface area measurements (BET method). Ion exchange capaci ty has been determined and effect o f heating on ion exchange capacity studied. The distribu­ti on behaviour towards several metal ions in different electrolyte med ia/concentrations has al so been studied . The utility of this material has bee n demonstrated by carrying out some binary metal ion separations. Further, as a case study, the separation/recovery of lead from an effluent containing lead has been demonstrated .

Tetravalent metal acid (tma) salts have found applica­tions in recovery of valuable products from industrial waste treatment' , removal of corrosion products2 and demineralization of water, in nuclear industry3, in analytical chemistry such as detection and estimation of metal ions4

, as impregnants on papers and glass5•6

,

in planar chromatograph/·8 and as adsorbents for pesticides!) etc. Earlier, Qureshi el ai. '0 have synthe­sized tin tungstate with tin to tungsten ratio 2: I and studied its ion exchange behaviour. However, the phases obtained were not stable (thermal/chemical) and materials did not possess good ion exchange capacity (0.50 meq. g" ). It was therefore thought of interest to synthesize a new phase of tin tungstate aimed at improving ion exchange capacity and stability both thermal and chemical.

In the present endeavour, a new phase of amor­phous tin tungstate (Sn W) of the class of tma salts has been synthesized by modified sol-gel method. The materi al has been characterized for elemental analy­sis, thermal analysis (TGA , DSC), spectral analysis (FfIR) and surface area measurements (BET method). The chemical resistivity of the material has been assessed in various acidic, basic and organic media. Ion exchange characteristics. such as ion exchange capacity has been determined and the effect

of heating on ion exchange capacity studied. Distri­bution behaviour towards several metal ions has been studied in water as well as in several electrolyte media/concentrations. On the basis of thi s study. distribution coefficient K" has been evaluated. Separation factor a for several metal ion pairs ha. been determined. The utility of the material is shown by achieving separation of Co(II) from Cu(II), Th(IV ) and Zn(IJ) . Finally, as a case study , the separa­tion/recovery of lead from an effluent containing lead has been demonstrated.

Experimental

Preparation of till tungstate (SII W) SnW was prepared by adding dropwise aqueous

solution of sodium tungstate (0.2 M , 100ml) to aqueous solution of tin tetrachloride (O.IM, 100ml ) with continuous stirring. The pH of the resulting solution along with the gel obtained was mai ntained at -2 pH and allowed to stand for at least 24h. filtered, washed with conductivity water till free of chloride ions and dried at room temperature. The dried material was brought to the desired particle size (30-60 mesh) and sorted by sieving. It was fin all y converted to the acid form by immersing in I M HN03, the acid being intermittently replaced , washed with conductivity water till free of acid and again dried at room temperature.

SnW was analyzed for tin and tungsten gravimetri ­cally as tin oxide" (cupferron method) and barium tungstate'2, respectively. Chemical resistivity of SnW was assessed in several mineral acids such as HC\. H2S04 and HNO), bases like NaOH and KOH and organic solvents such as alcohol , acetic acid and diethyl ether. Thermogravimetric analysi s was performed on Shimadzu DT-30 Thermal analyzer at a heating rate of lOoC min" . FfIR spectra were recorded on a BOMEM MB series with Epson Hi 80 printer/plotter using KBr wafer. Surface area of SnW was obtained by nitrogen adsorption BET method on a Carlo Erba Sorptomatic series 1800 at -196°C. x­ray diffractogram (28=5-90°) was performed on x­ray diffractometer Rigaku, Dmax-30 with Cu-Ko radiation and nickel filter.

560 INDIAN J CHEM, SEC A, MARCH 2003

Determination of ion exchange capacity The Na+ ion exchange capacity of SnW was deter­

mined by the column method l3 taking 0.5g of the acid treated ion exchanger in a glass column (int. dia 1 cm) fitted with glass wool at its bottom and passing 250 ml sodium acetate solution (0 .5M) through it at a flow rate of 0.5 ml min-I. The effluent was titrated against standard alkali solution to find out the total H+ ions eluted. The exchange capacity (in meq.g-I) was evaluated using the formula, av/w where a is molarity, v is the volume of alkali used during titration and w is the weight of Sn W. Further, the effect of heating on ion exchange capacity was also studied by heating several Ig portions of the sample for 2h at various temperatures in a muffle furnace and determining the Na+ exchange capacity by the column method at room temperature.

Distribution studies Distribution studies were carried out for several

metal ions by batch process 14. liOOmg of SnW in the H+ form was equilibrated with 20 ml of 0.001 M metal ion solution for 24h at room temperature. The metal ion concentration before and after sorption was determined by titration against standard EDTA. Distribution studies were also carried out for all the metal ions in different electrolyte me­dia/concentrations: 0.02 M and 0 .2 M of HN03,

HCI04. CH)COOH and NH4N03 • Distribution coefficient (Kd) was evaluated using the expression,

j-F V K =--x­

d F W

where j is the initial amount of the metal ion in the solution phase; F the final amount of the metal ion in the solution phase; V the volume of the metal ion solution (ml); W the weight of the ion exchanger.

Sepa ration of metal ions For binary separations, 2g of the material in H+

form was taken in a glass column (30 cmxl.lcm). The column was washed thoroughly with deionized water and the mixture of metal ions to be separated was loaded on it, maintaining a flow rate of 0 .5 m!. min-I. In all the separations, the cation concentration was maintained at 0.001 M and 5 ml of each metal ion solution was taken. The separation was achieved by pas. ing a suitable eluant through the column and the metal ions in the effluent were determined quantita­tively by EDT A titration.

The ini tial pH of the effluent cOn£21ining Pb was 6. This was adjusted to -2 pH by dropwise addition of 0.2M CH3COOH. This was essential as Pb is com­pletely sorbed in this electrolyte concentration . The separation of Pb from effluent has been carried out by the procedure as discussed above.

Results and discussion Chemical analysis of material indicates Sn:W rati o

to be 1 :2. TGA 'of SnW indicates 14% weight loss in the

temperature range of 100-180°C corresponding to the loss of external water molecules , after which a slow

change in weight is observed till 600°C. Thi s may be attributed to the condensation of structural hydroxyl groups.

DSC of Sn W shows only one exochermic peak at

-132°C which is attributed to the presence of water.

There is no endothermic peak upto 500°C which indicates that there is no phase change.

FTIR spectra of SnW shows broad bands in the region -3400 cm-I attributed to asymmetric and symmetric hydroxo -OH and aquo -OH stretches. A sharp medium band at -1600 cm-I is attributed to aquo (H-O-H) bending.

Chemical resistivity studies shows that Sn W is stable in concentrated mineral acids such as HCI (I N), HN03 (l4N) and H2S04 (36N) as evidenced by no change in colour, form or weight of the sample. It is also observed that Sn W is unstable in bases such as NaOH and KOH above 0.5 M concen tration . SnW is also found to be stable in ethanol, acetic acid and diethyl ether.

The surface area of Sn W is found to be 14.05 m2g-l

.

In the X-ray diffractogram, absence of any charac­teristic peak confirms the non crystalline nature of SnW.

Table I-Effect of heating on ion exchange capac ity o f SnW

Temperature Colour Ion exchange

capac ity meq.g-I

Room temp. White 1.52

100 Yellowish 1.08

200 Light brown 0.82

300 Brown 0.62

400 Brown 0.50

500 Brown 0.39

NOTES 561

100r------------------------------------------~--------,

90 ROOM Temp

100 C

300 ·C

_ 0 ---<ot---<>o- ~O O C

80- ------ - 200 C --~ 500 'C

70

60

50

:..: 40

30

10

0L-______ -=.-________ -. ________ ~----~--__.__ 4000 3300 2600 1900 1200 511

WAVENUMB ER em-I

Fig. I--FTIR spectra of SnW and SnW heated at various temperatures

Table 2--lon exchange capacity for alkali and alkaline earth metal ions

Metal ion Hydrated ionic Ion exchange capacity

radius (A 0)" (meq.g-I)

Li+ 10.00 1.50

Na+ 7.90 1.52

K+ 5.30 1.56 Mo 2+

<> 10.80 0.72

Ca2+ 9.60 1.10

8a+2 8.80 1.16

"Taken from Lange's hand book o f Chemistry, thirteenth ed ition

The Na+ ion exchange capacity of SnW is found to be 1.52 meq.g' l (as against the literature value 0.5 meq.g'I)IO. The effect of heating on ion exchange capacity (Table 1) reveals that ion exchange capacity decreases from l.52 to 0 .399 meq.g'l in the range

(l00-500°C) as temperature of heating increases. This is attributed to condensation of structural hydroxyl groups, bearing the exchangeable protons at higher temperature. This fact is also evident from the IR spectra of the heated samples of SnW (Fig. 1) It is observed that the intensity of the peaks at - 3400 cm' l and 1620 cm' l representative of the -OH group and external water molecules respectively diminishes as heating temperature increases.

The effect of size and charge of the exchanging ingoing ion on the i.e.c. of the material was studied for alkali and alkaline earth metals, the sequence shown by SnW is as follows and summarized in Table 2.

Table 3--Distribution coefficient of metal ions for SnW

Cation Form Kd(ml gil Zn2+ Acetate 378.08 Cu2+ Acetate 5703.33 Ni 2+ Acetate 596.07 Mn2+ Acetate 1141.1 7

C02+ Acetate 1850.00 Pb2+ Nitrate 2480.00 Hg2+ Ch loride 46.37

Bi3+ Nitrate 95 .77

La3+ Nitrate 104.00

Th4+ Nitrate 13.83

The sequence for alkaline earth metals is in accor­dance with the hydrated ionic radii . The ions with smaller hydrated ionic radii easily enter the cavities of the exchanger resu lting in higher exchange capacity. However, some irregularity that is observed in case of alkali metals is not uncommon with amorphous inorganic ion exchangers and has also been reported by other authors IS .

The distribution coefficient Kd calcu lated for sev­eral metal ions (Tables 3 and 4) reveals that Sn W has higher selectivity for Cu(ll), Pb(II) , Mn(I l) and Co(ll) while it has low selectivity for Th (IV). Pb(l)) is found to be completely sorbed on SnW in 0.2 M CH3COOH as electrolyte media. This may be due to the formation of lead acetate which is strongly adsorbed on SnW. Further, it has been observed that Kd values for metal ions such as Zn(JI), Cu(II ). Mn(IJ), Pb(II), Bi(III), La(III) and Th(lV) are less in aqueous media as compared to the various electrolyte

562 INDIAN J CHEM, SEC A, MARCH 2003

Table 4--Effect of e lectrolyte concentration on the Kd values of metal ions for Sn W

Metal Kd(ml g. l)

Ion Di stilled 0.02M 0.2M 0.02M 0.2M 0.02M 0.2M O.O.2 M 0.2M

water HNOJ HN03 HCI04 HCI04 CH3COOH CH3COOH NH4N03 NH4N01

Z ,+ n 378.08 112.59 15.85 116.10 4.35 491.98 348.05 176.78 42 .52 Cu'+ 5703.33 229.40 14.66 181.07 27.56 1088 .00 15900 531.8 1 106.66 Ni'+ 596.07 107.57 N.S. * 78.08 N.S.* 588.4 1 341.33 227.65 51.25 Mn'+ 114 1.1 7 123.40 16.62 121.12 23 .52 1295.48 547.54 359.69 13.05 C02+ 1850.00 164.44 77.96 64.50 337.84 275.46 620.00 44~..38 n:n Pb2+ 2480.00 3820.00 464.50 3302.27 673. 19 3454.54 C.S.* 4043.33 18 10.00 Hg2+ 46.37 9.22 17.25 46.37 17.96 39.43 23.68 28 18 34.48 Bi 3+ 95.77 100.01 111.27 123.43 11 3.63 11 2.45 119.69 113.63 11 4X\ La3+ 104.00 89.52 12.58 130.47 43.20 135.9 1 222.22 15_.53 149.42 Th4+ 13.83 17.25 5.43 12.50 1.78 15. 18 13. 16 15 18 15.1 8

" N. S.- No sorpti on

'" C. S.-Complete sorpti on

Table 5--Binary separation of metal ions on SnW

Separation a Eluant

ach ieved

Co'+ - Cu'+ 3.08 0.2 M HN03 (Cu)

0.2 M HCI04 (Co)

Co2+ _ Th4+ 133.76 0.2 M HC I0 4 (Th )

I M NH4N03 (Co)

Co2+ _ Zn'+ 4.89 0.2 M HC I0 4 (Zn)

I M NH4N03 (Co)

K a=-" I

K dl

media. Simi lar observations have been made by other workers. Sharma el al. 16 have studied ion exchange behaviour of stannic tungs tate silicate and observed that, K" values for Mn(II) , Ni ( lJ) , Pb(II) , Co(ll) and AI(III ) were hi gher in 0.1 M HNO) as compared to aq ueous medi a. Varshney and coworkers l7 have also observed that in case of tin (IV) antimonite, Kd value of CoO[) is higher in I M HCI04 as compared to aqueous media. They also synthes ized po[yacryloni­trile thorium (IV) phosphatel 8 and found that K, va lues for several alkaline earth metals and transition metals . how same behav iour.

The K" values suggest the poss ibilities for many

important binary separations. Separation factor a has b~en calculated for the possible metal ion separations (Table 5) .

Binary separations of metal ions in three cases were carried out:

Metal ion (g) Efficiency

Loaded Eluted (%)

3.07x I0-4 2.8 Ix I0·4 91.66

2.44x I 0-4 2.36x 10-4 96.36

9.39x lO·4 8.8 Ix I0·4 93.82

2.44x I 0-4 2.30x I0·4 93.95

3.80x I 0-4 3.44x I 0-4 90.58

2.44x I 0-4 2.24x I0·4 9 1.69

Co(lI) - Cu(II) , Co(ll) - Th(lV) and Co(l [) - Zn (II ).

In case of Co(ll) - Cu(II), 0.2 M HCI04 is used as el uant for Co(ll) whi le 0.2 M HNO) is used fo r C u(II ). In case of Co(ll) - Th(IV) and Co(l[) - Zn(l l), I M NH4NO) is used to e lu te Co(ll) and 0.2 M HClO~ is used to e lute Th(lV) and Zn(ll) . SnW ex hibits 90-96% efficiency for these metal ion separations.

The most promising property of the materi a l Sn W is its high selectivi ty for lead. Pb has been eluted by I M HNO), where 5 1 % of Pb is recovered . The lower recovery of Pb may be due to the formation of an insoluble complex of lead. Lead selective tma salt s have been reported by many worker s. Varshney and coworkers have reported lead selec ti ve po lyacry loni­trile thorium (IV) phosphate lS and tin (IV) anti-

17 Q h' / 19 h d . . monate . ures I et a . ave reporte tltanlUIll molybdate specific for lead. Such a high affinity for

NOTES 563

Pb shows great promise In the field of pollution chemi stry where lead needs to be separated fro m other pollutants.

Conclusion The new phase of tin tungstate prepared exhibits

high value of ion exchange capacity as against the value reported eariier'o. ]t also exhibits high selectiv­ity for several metal ions and shows 90-96% effi­ciency for the sep::o ration of CoOl) from Zn(II) , Cu(II) and Th(lV). The effective separation/recovery of lead from effluent containing lead makes SnW a potential candidate in the field of pollution chemistry, where an effect ive separati on method is needed for lead from other pollutants. The above study reveals that Sn W exhibits the characteristics of a promising ion ex­changer.

Acknowledgement Thanks are due to the Head, Department of Ap­

plied Chemistry, for providing necessary laboratory facilities.

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