Poly(styrene-p-hydroxamic acids): synthesis, and ion exchange separation of rare earths

Reactive & Functional Polymers 39 (1999) 155–164

Poly(styrene-p-hydroxamic acids): synthesis, and ion exchange separation of rare earths Y.K. Agrawal*, H. Kaur, S.K. Menon Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad 380009, India Received 15 January 1997; received in revised form 21 October 1997; accepted 24 November 1997

Abstract Six new poly(styrene-p-hydroxamic acids) have been synthesised, characterised and their physico-chemical properties have been determined. These polymers are used as chelating ion-exchange resins for the separation and determination of rare earths. The method is used for the determination of lanthanum, cerium, neodymium and yttrium in the synthetic, standard and environmental samples.  1999 Elsevier Science B.V. All rights reserved. Keywords: Poly(styrene hydroxamine acids); Rare earth; Ion exchange

1. Introduction During the last two decades, there has been an increased interest in the synthesis and analytical applications of chelating ion exchangers. These are widely used for the pre-concentration of trace elements from various materials and also for selective chromatographic separation of several metal ions. Chelating ion exchangers bearing a hydroxamic acid functional group have found wide applications owing to their favourable properties. The study of poly(hydroxamic acids) was pioneered by Deuel and co-workers [1,2]. They prepared hydroxamic acid derivatives of Amberlite IRC50 and other poly(methacrylic acid) resins. Since then a large number of poly(hydroxamic *Corresponding author. Tel.: 191-79-6440969; fax: 191-796441654.

acids) derived from poly(amidoximes), poly(acrylonitrile), poly(styrene-co-maleic acid) and poly(butylacrylate) have been synthesised. These have been used for several successful analytical separations of Ag(I) and Au(II), binary and ternary mixtures of Fe(II), Cu(II), Pb(II), Ni(II) and Th(IV), U(VI), Ti(IV) by various workers [3–11]. Fetscher has reported a radioactive source consisting of Pu, U, Co and Th immobilised on a poly(hydroxamic acid) [12]. A survey of the literature reveals that not much work has been done on the synthesis of polyhydroxamic acids derived from polystyrene moiety. So far no attempt has been made for separation of lanthanides using these ion exchangers. In this paper we describe the synthesis of N-substituted poly(styrene-p-hydroxamic acids) derived from polystyrene and separation of lanthanides. Various parameters like pH,

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kinetics of sorption, pre-concentration and effect of electrolytes were studied.

ml) and refluxed on a water bath for 5 h. The reaction mixture was allowed to cool, filtered and washed with dry diethylether: yield, 85%.

2. Experimental

2.2.4. Hydroxylamines The N-phenyl-, N-p-tolyl-, N-p-chloro- and N-m-chlorophenyl hydroxylamines were prepared from their respective nitro benzenes and N-methyl hydroxylamine from nitromethane using zinc dust and NH 4 Cl as described elsewhere [17–21].

2.1. Chemicals and apparatus All the chemicals used were of A.R. grade of B.D.H. unless otherwise specified. The solvents were purified by the method of Weissberger et al. [13]. Polystyrene (average mol. wt. 10 5 ) was used as such. The metal ion solution of Ce(IV) was prepared using ceric ammonium nitrate. Other lanthanide solutions were prepared from their oxides. Metal ion solutions were standardized spectrophotometrically [14,15]. The IR spectra were recorded on a PerkinElmer 1401 ratio recording infrared spectrophotometer as KBr pellets. Ciba–Corning 2800 UV-visible spectrophotometer was used for spectral measurements.

2.2. Synthesis 2.2.1. p-Acetyl polystyrene Electrophilic substitution of polystyrene (20 g) with acetyl chloride (22 ml) was carried out by the Friedal–Crafts reaction using AlCl 3 (53.6 g) in 200 ml carbon disulphide. The yellow solid obtained after acetylation was dissolved in acetone and precipitated by pouring in the water with vigorous stirring: yield, 87% [16]. 2.2.2. p-Carboxy polystyrene To p-acetylpolystyrene (6 g) was added 12% aqueous solution of sodium hypochlorite, refluxed for 12 h and it was acidified, white solid separated by filtration and washed with water until washings are free from chloride ions [16]: yield, 80%. 2.2.3. p-Chloroformoyl polystyrene p-Carboxy polystyrene (7.59) was mixed with thionyl chloride (7.5 ml) and dioxane (5

2.2.5. Poly(styrene-p-hydroxamic acids) Into a three-necked flask equipped with stirrer, thermometer and dropping funnel freshly prepared N-hydroxylamines (0.05 mol) in 50 ml diethylether along with a suspension of sodium bicarbonate (0.075 mol) in 10 ml water were stirred at 08C. Solid p-chloroformoyl polystyrene was added in small amounts over a period of 45 min. Stirring was continued for further 3 h. The solid was filtered, washed with diethylether to remove unreacted hydroxylamines and dried over P2 O 5 . The dried poly(hydroxamic acids) were sized to 50–100 mesh. 2.2.6. Purification of poly(styrene-phydroxamic acids) The poly(styrene-p-hydroxamic acids) were kept in contact with 4 M HCl for 24 h, then washed with distilled water repeatedly until the filtrate is free from chloride ions before using as ion exchangers. 2.2.7. Hydrogen ion capacity The hydrogen ion capacity of the poly(styrene-p-hydroxamic acids) was determined by back titration of a mixture of 100 mg of polymer and 10 ml of 0.1 M sodium hydroxide after equilibrating the mixture for 24 h [22]. 2.2.8. Sorption capacity for metal ions using batch technique The metal sorption capacities for the metal ions La(III), Ce(III), Ce(IV), Nd(III) and Y(III)

Y.K. Agrawal et al. / Reactive & Functional Polymers 39 (1999) 155 – 164

were determined by equilibrating 1 g of the poly(styrene-p-hydroxamic acids) with 50 ml of 0.005 M solutions in the pH range of 2–6 [22]. The residual concentrations of metal ions were determined after 24 h.

2.2.9. Kinetics of sorption The rate of uptake of metal ions was determined by shaking 1 g of the polyhydroxamic acid with 50 ml of metal ion solution (0.001 M) and the pH adjusted to 6.0 [22]. Two-ml aliquots of solution were withdrawn at the intervals of 5, 10, 20, 40 and 80 min and the residual concentrations of metal ions were determined. The distribution coefficients at 6.0 pH were determined by batch experiments in which a known quantity of polyhydroxamic acid is shaken with a solution containing known concentration of metal ions followed by analysis of the two phases after equilibrium was attained. amount of metal/g of poly(styrene-p-hydroxamic acid) K 5 ]]]]]]]]]]] amount of metal per ml of the solution

The rate constant is calculated [23]: log

F

G F

Xe aKt ]]] 5 ]]] Xe 2 X 2.303Xe

G

where a5concentration of the ions originating from the exchangers which are regarded as dissolved in the chemical reactant sense; Xe 5 equilibrium concentration in solution of the ion originally in the exchanger, K5reaction rate constant. The fraction attainment of equilibrium is given by X /Xe while the fractional equilibrium conversion of the resin is equal to Xe /a.

2.3. Sorption studies using column A glass column 4.63150 mm was packed with poly(styrene-p-hydroxamic acid) using slurry packing technique. The column was

157

conditioned to desired pH by passing buffers equal to 10 bed volumes at a rate of 1.0 ml / min. For the breakthrough studies metal solution (2.5 mmol / l) were passed through the column at the flow-rate of 0.5 ml / min. Ten-ml aliquots of effluent were collected and analysed for the concentration of metal ions. 3. Results and discussion The hydrogen ion capacity of p-carboxy polystyrene intermediate was 5.01 mmol / g and those of poly(styrene-p-hydroxamic acids) are in the range of 2.72–4.53 mmol / g (Table 1). The infrared spectra of p-carboxy polystyrene intermediate showed distinct bands at 1710 cm 21 , a very broad band at 2700–3500 cm 21 and a band at 1290 cm 21 which account for nC=O . nO – H . and nC – O , stretching vibrations, respectively. In the chloroformoyl polystyrene intermediate the broad band above 2700 cm 21 disappears and nC=O band at 1710 cm 21 shifts to 1780 cm 21 . In the poly(styrene-p-hydroxamic acids) the nC=O bands were between 1610 and 1655 cm 21 and nO – H band between 3200 and 3250 cm 21 . Presence of a single sharp carbonyl band in substituted poly(styrene-p-hydroxamic acids) indicates almost complete conversion to hydroxamic acid group. The presence of hydroxamic acid group was further confirmed by quantitative tests with Fe(III) and V(V) [17]. The physicochemical properties of substituted poly(styrene-p-hydroxamic acids) are shown in Table 1. The moisture content, true density, apparent density (wet and dry), water retention were determined by methods described elsewhere [22]. The moisture content (%) in the various substituted acids varies between 2.58 and 6.49%. The poly(styrene-p-hydroxamic acid) compound (I) has the highest moisture content. The true wet densities of the ion exchangers are in the range of 1.19–1.68. The swelling or water retention in the range of 1.12–1.35. All these values suggest that polymer structure remains rigid and these can be

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158

Table 1 Physicochemical properties of the poly(styrene-p-hydroxamic acids)

R5H, C 6 H 5 , CH 3 , p-Cl-C 6 H 4 , m-Cl-C 6 H 4 or p-CH 3 -C 6 H 4

Unsubstituted (I) Solid (%) Moisture (%) True wet density (g / cm 3 ) Apparent density (g / cm 3 ), dry Apparent density (g / cm 3 ), wet Swelling (W.R. / g) Void volume Sodium hydrogen exchange capacity (mmol / g) Concentration of ionogenic groups (meq / cm 3 ) IR (cm 21 ) nC=O nO=H % found C % found H % found N

N-methyl (II)

N-phenyl (III)

93.5 6.49 1.47 0.44 0.25 1.45 0.82 4.53

97.42 2.58 1.68 0.49 0.34 1.21 0.79 4.23

97.39 22.61 1.26 0.35 0.24 1.12 0.80 3.05

96.76 3.24 1.27 0.29 0.23 1.21 0.81 2.72

96.90 3.10 1.21 0.27 0.22 1.18 0.81 2.74

97.19 2.81 1.19 0.31 0.24 1.06 0.80 2.96

1.12

1.45

0.74

0.64

0.61

0.68

1610 3200 69.13 5.87 5.86

1620 3250 67.34 5.29 4.27

1655 3250 61.16 4.51 3.79

1650 3250 61.15 4.52 3.79

1610 3200 68.10 6.63 4.10

1610 3200(vb) 67.99 59.38 6.35

used for column studies. Ion exchangers were found to be thermally and chemically stable above 2008C and up to 5 M HCl. Since these resins having the acidic functional hydroxamic group have a tendency to hydrolyse at the higher concentration of alkali (.6 N NaOH). Metal ion capacities for the lanthanides studied showed linear increase between pH 2 and 6 (Fig. 1). Study at higher pH range was avoided since lanthanides tend to hydrolyse above 6 pH. Unsubstituted poly(styrene-p-hydroxamic acid) showed maximum capacity and highest Kd values. In substituted poly(styrene-phydroxamic acids) the capacities increase in order of p-tolyl,N-phenyl,p-chlorophenyl, m-chlorophenyl,N-methyl. On the basis of highest capacity unsubstituted poly(styrene-phydroxamic acid) (I) was taken for column studies. The plot of log Kd values against the pH is shown in Fig. 2. The rate constants have been

N-p-chlorophenyl (IV)

N-m-chlorophenyl (V)

N-p-tolyl (VI)

given in Table 2. Void volume for the column was 0.82. All the breakthrough curves were steep at the breakthrough point (Fig. 3) t 1 / 2 for all the metal ions was ,5 min. No specific selectivity for any of the metal ions was shown by these ion exchangers (Tables 2 and 3). However, a small increase in exchange capacity and Kd values with increase in the atomic weight is observed in case of La(III), Ce(III), Nd(III) (Table 2). Cerium(IV) is not absorbed up to pH 8. At pH 9 its sorption capacity from citrate solutions is 0.1 mmol / g.

3.1. Preconcentration of lanthanides and effect of various electrolytes Preconcentration by a factor of 100 was achieved in all the cases from trace metal ion solutions (3 ppm) at pH 6.0. The metal ions were eluted with 0.1 M HCl. Effect of various

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159

Fig. 1. Exchange capacity as function of pH on poly(styrene-p-hydroxamic acid).

electrolytes such as NaCl, Na 2 SO 4 , NaNO 3 , Na 2 CO 3 on metal absorption are shown in (Table 4).

neodymium(III) and yttrium(III) were also achieved (Fig. 4).

3.3. Separation of La( III) from Ce( III) 3.2. Separation of Ce( IV) from La( III), Nd( III), Ce( III), Yt( III) The batch studies and pH of sorption reveal that the separation of Ce(IV) from other trivalent lanthanides can be achieved. A mixture of La(III) and Ce(IV) was passed through the column at pH 6. La(III) was retained by the column and Ce(IV) eluted out and estimated spectrophotometrically. Lanthanum(III) was eluted from the column using 0.1 M HCl. Similar separations of cerium(IV) from

A separation of La(III) from Ce(III) was obtained by selective elusion of the La(III) with 0.5% ammonium oxalate solution (Fig. 5).

3.4. Separation of lanthanum from other metal ions [ Fe( III), Al( III), Cu( II), Pb( II), Mg( II), Ca( II), Be( II)] By judicious choice of pH it was possible to separate lanthanum from many of the transition and alkaline earth elements. Lanthanum is sepa-

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160

Fig. 2. log Kd vs. pH of La, Ce, Nd, Y.

Table 2 Distribution constants and half-life periods of various metal ion on (I) at pH 6 Metal

t 1 / 2 (min)

Kd

K

La 31 Ce 31 Nd 31 Y 31

,5 ,5 ,5 ,5

4472 5228 5464 6639

6.8310 24 7.2310 24 7.3310 24 7.6310 24

s 21 s 21 s 21 s 21

rated from Fe 31 , Al 31 , Cu 21 , Pb 21 by passing the mixtures of Lanthanum and Fe 31 , Al 31 , Cu 21 , Pb 21 at pH 2, Lanthanum is eluted out,

whereas others metals were retained in the column. Fe 31 , Al 31 and Cu 21 were eluted out of the column by 4 M HCl, whereas Pb 21 was eluted with 0.5 M HNO 3 . The concentrations of the ions were measured by AAS. Similarly passing the mixture of lanthanum and alkaline earth, elements (Be 21 , Mg 21 , Ca 21 ) through column at pH 4, Be 21 , Mg 21 and Ca 21 were eluted and lanthanum was retained in the column. The lanthanum was eluted with 0.596 ammonium oxalate and its concentration was measured by AAS.

Y.K. Agrawal et al. / Reactive & Functional Polymers 39 (1999) 155 – 164

161

Fig. 3. Break through curves on poly(styrene-p-hydroxamic acid).

Table 3 Total sorption capacities of various metal ions on poly(styrene-p-hydroxamic acids) Metal

La 31 Ce 31 Nd 31 Y 31

pH

6.0 6.0 6.0 6.0

Sorption capacity (mmol / g) H (I)

N-methyl (II)

N-phenyl (III)

p-Chlorophenyl (IV)

m-Chlorophenyl (V)

p-Tolyl (VI)

0.60 0.62 0.66 0.69

0.47 0.50 0.52 0.54

0.37 0.39 0.41 0.43

0.36 0.38 0.40 0.42

0.38 0.39 0.42 0.42

0.35 0.36 0.38 0.41

3.5. Determination of rare earths in standard and environmental samples

Table 4 Effect of electrolytes on metal sorption Serial no.

Electrolyte

Tolerance limit

1 2 3 4

NaCl Na 2 SO 4 NaNO 3 Na 2 CO 3

2.5 1.0 2.0 1.0

M/I M/I M/I M/I

The present chromatographic method is applied for the separation and determination of lanthanum, cerium and yttrium in synthetic, monazite sand and sea water samples.

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Fig. 4. Separation of Ce / La (a), Ce / Nd (b) and Ce / Y (c).

The known quantity of the monazite sand (0.5 g) was digested with concentration HCl and concentration HNO 3 (20:1) and diluted to 100 ml with distilled water. The appropriate aliquot

was passed through the column after judicious adjustment of the pH. The contents after the separation were analysed by MS. The results are given in (Table 5).

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163

Fig. 5. Separation of Ce 31 / La 31 .

Table 5 Determination of lanthanum, cerium, neodymium and yttrium in the synthetic, monazite sand and sea water samples Serial no.

I II

III

Samples

Concentration (ppm)

a

Synthetic mixture Sea water b Bombay Malad Bombay Thana Bombay Monazite Sand

La

Ce

Nd

Y

9.9860.1

9.9960.1

5.0260.2

4.9560.2

5.260.5 6.060.4 23.9% (23.87%)

6.560.3 9.860.5 23.65% (23.75%)

1.560.3 3.560.5 — —

2.860.2 3.260.5 0.32% 0.36%

The standard values are given in parenthesis, average of the 10 determinations. a The mixture contains La (10 ppm), Ce (10 ppm), Nd (5 ppm) and Y (5 ppm) as standard; average of the eight determinations. b Sea water samples from Bombay sea, average of the 15 determinations.

Acknowledgements One of the author (H.K.) is grateful to UGC for awarding the JRF.

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