Sorption of Fe(III) containing ions on strongly basic anion exchangers AV-17 and Varion-AD

Reactive & Functional Polymers 46 (2001) 203–211 www.elsevier.com / locate / react

Sorption of Fe(III) containing ions on strongly basic anion exchangers AV-17 and Varion-AD V. Gutsanu*, Raisa Drutsa, V. Rusu Technical University of Moldova, 168, Bl. Stefan cel Mare, MD-2004 Chisinau, Moldova Received 21 June 1999; received in revised form 3 May 2000; accepted 10 June 2000

Abstract It is shown that sorption isotherms of Fe(III) containing ions from Fe 2 (SO 4 ) 3 solutions on the strongly basic anion exchangers AV-17 and Varion-AD are well described by Langmuir’s equations. Almost all sorption centers of the polymers are energetically homogeneous towards Fe(III) containing ions. The maximum temperature dependence of sorption was ¨ found to be about 508C. The Mossbauer spectra of the Varion-AD retaining Fe(III) containing ions from solution at 30 and 508C showed the existence of the Fe(III) ions in a single electronic state, i.e., there was an absence of different kinds of Fe(III) compounds. The sorption of Fe(III) containing ions on the polymers essentially decreases with increasing the ionic strength of the Fe 2 (SO 4 ) 3 solution on adding calculated amounts of KNO 3 , NaNO 3 , NaClO 4 or Na 2 SO 4 .  2001 Elsevier Science B.V. All rights reserved. ¨ spectroscopy; Jarosite Keywords: Strongly basic anion exchangers; Fe(III) containing ions sorption; Isotherms; Mossbauer

1. Introduction In a previous study [1,2] it was reported that crosslinked ionic polymers containing strongly basic groups (R 4 N 1 ) are able to retain Fe(III) containing ions from Fe 2 (SO 4 ) 3 solutions. It was suggested [3], that the sorption of Fe(III) containing ions takes place through formation, in the polymer phase, of the mineral jarositetype compounds A[Fe 3 (SO 4 ) 2 (OH) 6 ], where A 1 1 1 1 may be R 4 N , H , K , Na and other cations ¨ [4]. Mossbauer spectroscopy investigations showed [3,5] that these Fe(III) containing compounds, while in the phase of R 4 N 1 containing *Corresponding author. Fax: 1373-2-494-050.

polymers, are in the form of solid ultrafine particles in a superparamagnetic state. According to Suzdalev [6], the size of the particles can be estimated to be about 3–30 nm. The existence of the Fe(III) containing compounds in the polymer phase influences their sorption properties. The strongly basic anion exchangers are selective for NCS 2 , CN 2 and NCO 2 anions [3]. Retention of Fe(III) ions is often an uncontrollable process that contaminates anion-exchange materials during use in industry. This is why there is an interest in investigating the sorption of Fe(III) containing ions on strongly basic anion exchangers and their dependence on different factors for researchers using these exchangers.

1381-5148 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 00 )00064-X

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In the present paper Fe(III) ions sorption isotherms and the influence of temperature and ionic strength of the solutions on Fe(III) containing ions sorption on the strongly basic anion exchangers AV-17 [containing RN 1 (CH 3 ) 3 groups] and Varion-AD [containing 1 RN (CH 3 ) 2 CH 2 CH 2 OH groups] are discussed.

2. Experimental Commercial styrenedivinylbenzene strongly basic anion exchangers, AV-17 and Varion-AD, in their Cl 2 forms were used. Their full exchange capacity is, respectively, 3.5–4.5 and 4.0 mg eq / g [7]. Dried samples (0.2 g) of the polymers were placed in contact with 200 ml of Fe 2 (SO 4 ) 3 solution for 24 h, which was enough for equilibrium to be established. The ratio of polymer mass and the contact solution volume allowed us to consider the equilibrium concentration of Fe(III) ions in the liquid phase as well as the initial one. The pH of the solution – the sample systems were maintained at 2.060.1 by using either H 2 SO 4 or KOH solutions. Following contact, the samples were filtered, washed with distilled water and analyzed for their iron content. The sorption isotherms were obtained at 508C in Fe 2 (SO 4 ) 3 solutions, ranging in concentration from 0 to | 2.4 3 10 22 M. The temperature dependence of the sorption of Fe(III) containing ions in 2 3 10 22 M Fe 2 (SO 4 ) 3 solutions was investigated over the temperature range of 20–708C. The system temperature was maintained at a constant with an error of 618C. There was no evidence of the resins degrading at these temperatures. The influence of the ionic strength of the solutions on the sorption of Fe(III) containing ions was investigated at 30 and 508C by adding calculated amounts of KNO 3 , NaNO 3 , NaClO 4 or Na 2 SO 4 to the 2 3 10 22 M Fe 2 (SO 4 ) 3 solution. The Fe(III) content of the samples was determined photocolorimetrically, with error of 60.3 mg Fe / g, using a,a9-dipyridine after

desorption of the ion with a solution of 1–1.5 M HCl. Some samples of the Fe(III)-containing Var¨ ion-AD were investigated by Mossbauer spec¨ troscopy at 300 and 80 K. The Mossbauer spectra were obtained on an ICA-70 spectrometer in an accelerated dynamic regime, employing 57 Co in a Cr matrix as a g-ray source. The ¨ parameters of the Mossbauer spectra, such as isomeric shift (d ), quadrupole splitting (DEQ ), left line width (Gl ) and right line width (Gr ) of the doublets with an error of 60.04 mm s 21 were referenced to sodium nitroprusside.

3. Results and discussion

3.1. The temperature influence on Fe( III) ions sorption As is shown in Fig. 1 that the sorption of Fe(III) containing ions from the Fe 2 (SO 4 ) 3 solution on strongly anion exchangers AV-17 and Varion-AD in dependence of temperature passes through a maximum at about 508C. The first important conclusion from the temperature dependence of the Fe(III) containing ions retaining on polymers is that the sorption is not a physical but a chemical process. This fact excludes ion exchange as a possible process for

Fig. 1. Temperature dependence of the Fe(III) containing ions sorption on anion exchangers AV-17 and Varion-AD.

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the sorption of Fe(III) containing ions. It also excludes sorption due to the formation of a Fe(OH) 3 precipitate in the polymer phase. As it is known [8], the degree of hydrolysis of ions in the exchange phase differs from that in solution. We suggest that the retention of the Fe(III) containing ions on the strongly basic anion exchangers is due to the formation of jarositetype compounds in the polymer phase. The appearance of a maximum on the temperature dependence of sorption shows that in the polymer–Fe 2 (SO 4 ) 3 solution system processes occur with opposing influences on sorption. These processes could take place in the polymer phase or in solution. To detect the existence of different Fe(III) compounds in the polymer phase we investigated Fe(III) containing samples of Varion-AD obtained at 30 (conditions I and II) and 508C (conditions III and ¨ IV) using Mossbauer spectroscopy. The re¨ sulting Mossbauer spectra are presented in Fig. 2 and their parameters are listed in Table 1. They show that in the polymer phase there are only Fe(III) ions in the high spin state. A visible amount of Fe(II) ions is not detected, which indicates the absence of reducing centers in the polymers. The electronic state of Fe(III) ions in Varion-AD is almost the same as in AV-17 [3]. Perhaps some of the OH groups of Varion-AD coordinate with Fe(III) ions affect the symmetry of the compound. The shape of the spectra is important along with their characteristic values, showing that in the polymer phase there is only one type of Fe(III) compound. So the Fe(III) containing ions sorption temperature dependence maximum (Fig. 1) is probably a result of the change which takes place in the Fe 2 (SO 4 ) 3 solution on heating. It is 31 known that in aqueous solutions Fe ions hydrolyse. The presence of some of such compounds as Fe 31 , FeOH 21 , Fe 2 (OH) 241 , Fe 3 (OH) 451 , 1 2 Fe(OH) 2 , Fe(OH) 3 and Fe(OH) 4 will effect the influence of the concentration of the iron salt, pH, temperature and ionic strength of the solution on the sorption properties.

205

¨ Fig. 2. Mossbauer spectra of the anion-exchanger Varion-AD after retaining Fe(III) containing ions from Fe 2 (SO 4 ) 3 solution at 30 (I, II) and 508C (III, IV).

According to Refs. [9,10], under the conditions used in our experiments equilibrium (1) is established: 2[Fe(H 2 O) 6 ] 31 ⇔2[FeOH(H 2 O) 5 ] 21 1 2H ⇔[Fe 2 (OH) 2 (H 2 O) 8 ] 1

41

1

1 2H 1 2H 2 O (1)

The relative content of Fe(III) containing ions in the solution we used, according Refs. [9,10] 31 may be estimated as about 25% [Fe(H 2 O) 6 ] , 21 50% [FeOH(H 2 O) 5 ] and 25% [Fe 2 (OH) 2 (H 2 O) 8 ] 41 . Not all of these cations are able to participate

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Table 1 ¨ Parameters of the Mossbauer spectra of anion-exchanger Varion-AD retaining Fe(III) containing ions from Fe 2 (SO 4 ) 3 solutions at different temperatures

d (60.04 mm / s)

DEQ (60.04 mm / s)

Gl (60.04 mm / s)

Gr (60.04 mm / s)

Sample obtained at 308 C 300 80

0.56 0.66

0.71 0.86

0.62 0.60

0.69 0.7

Sample obtained at 508 C 300 80

0.65 0.54

0.77 0.79

0.68 0.63

0.64 0.7

T (K)

in the formation of the Fe(III) compounds in the exchanger phase. With increasing pH, the concentration of [Fe(H 2 O) 6 ] 31 cations in solution decreases, but sorption of the Fe(III) ions on R 4 N 1 containing polymers grows [3]. From solutions of pH#1.5, the sorption of Fe(III) ions is absent. So [Fe(H 2 O) 6 ] 31 cations do not take part in the formation of Fe(III) compounds in the polymer phase. With an increase in temperature, equilibrium (1) shifts to the right and in the absence of precipitate formation the concentration of [Fe 2 (OH) 2 (H 2 O) 8 ] 41 cations in solution increases [10]. But as is seen in Fig. 1 at t.508C sorption of Fe(III) containing ions on the polymers decreases. Therefore the participation of the [Fe 2 (OH) 2 (H 2 O) 8 ] 41 cations in the Fe(III) compounds formation in the polymer phase is unlikely. On heating, the 41 [Fe 2 (OH) 2 (H 2 O) 8 ] cations are transformed into [(H 2 O) 5 Fe–O–Fe(H 2 O) 5 ] 41 ions. These dimeric cations cannot easily be restructured to form new units [11], and they have not been ¨ detected in the polymer phase by Mossbauer spectroscopy. During the formation process of the Fe(III) compounds in the polymer phase it is probable that the [FeOH(H 2 O) 5 ] 21 cations participate. In investigating the effect of pH, the concentration of the [FeOH(H 2 O) 5 ] 21 ions in solution was seen to pass through a maximum [9]. The position and value of the maximum depends also on the concentration of iron ions, ionic strength and temperature. The degree of hydrolysis of Fe 31 ions in the polymer phase

differs from the one in solution. In the polymer phase the concentration of SO 22 4 ions is much higher than in the contact solution and the 1 2 equilibrium SO 22 may estab4 1H ⇔HSO 4 lished. The hydrolysis processes taking place in the resin may result in local changes in the [H 1 ]:[OH 2 ] ratio, creating conditions for compounds to be formed according to Scheme (2) 3[FeOH(H 2 O) 5 ] 21 1 3OH 2 1 2 2SO 22 4 ⇔ [Fe 3 (SO 4 ) 2 (OH) 6 ] 1 15H 2 O

(2)

The complex anions polymerizes themselves to form a solid-phase [12]. Equilibrium (2), and therefore the formation of jarosite type compounds, does not take place in the liquid phase of the systems we were using. The conditions of the formation of jarosite-type compounds in solution are described in Refs. [12,13].

3.2. The influence of the ionic strength of solution on Fe( III) ions sorption When the strongly basic anion exchangers are used in industry, they are usually in contact with solutions of various composition. Therefore in the investigations of Fe(III) containing ions sorption on such polymers, the influence of solution composition is of interest. We have studied the influence of the ionic strength of solutions on Fe(III) containing ions sorption using KNO 3 , NaNO 3 , NaClO 4 and Na 2 SO 4 salts. As is shown in Figs. 3–6 the

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Fig. 3. Influences of the ionic strength of solutions formed by adding KNO 3 on Fe(III) containing ions sorption on anion exchanger AV-17.

Fig. 4. Influence of the ionic strength of solutions formed by adding NaNO 3 on Fe(III) containing ions sorption on anion exchanger AV-17.

sorption of Fe(III) containing ions on AV-17 essentially decreases on adding these salts to a 2310 22 M Fe 2 (SO 4 ) 3 solution. At the same ionic strength, the sorption of the metallic ions from solution is greater at 508C than at 308C. As well as increasing the temperature, increasing the ionic strength of solution displaces equilibrium (1) to the right [10]. When exchangers AV-17 and Varion-AD come into contact with Fe 2 (SO 4 ) 3 solutions

207

Fig. 5. Influence of the ionic strength of solutions formed by adding Na 2 SO 4 on Fe(III) containing ions sorption on anion exchanger AV-17.

Fig. 6. Influence of the ionic strength of solutions formed by adding NaClO 4 on Fe(III) containing ions sorption on anion exchanger AV-17.

equilibrium (3) is established more rapidly than equilibrium (2): 2R 4 NCl 1 SO 4 ⇔(R 4 N) 2 SO 4 1 2Cl 2-

2

(3)

when R 4 N 1 are the functional groups of the polymers. So the formation of jarosite-type compounds takes place inside the anion exchangers in the 2 form of SO 22 4 / Cl . On adding KNO 3 , NaNO 3 and NaClO 4 salts to the Fe 2 (SO 4 ) 3 solution

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contact with resins also takes place and processes (4) and (5) are set up: 2 R 4 NCl 1 NO 2 3 ⇔R 4 NNO 3 1 Cl

(4)

R 4 NCl 1 ClO 42 ⇔R 4 NClO 4 1 Cl 2

(5)

The rate of the processes (4) and (5) are more than of the Fe(III) containing ions sorption too. Although the selectivity of anion sorption on the 2 2 2 exchangers is SO 22 4 .ClO 4 .NO 3 .Cl 22 [7,14], the content of SO 4 ions in the resin phase falls with increasing the ionic strength of the solutions. Therefore the Fe(III) containing ions sorption sharply decreases as the ionic strength of solution increases. Another case is when Na 2 SO 4 is added to the Fe 2 (SO 4 ) 3 solution in contact with a polymer. This causes the concentration of (R 4 N) 2 SO 4 groups to grow, and the resulting sorption of Fe(III) ions at 508C in a large measure reflects the influence of the ionic strength of the solution. The sorption of Fe(III) containing ions on strongly basic anion exchangers also depends on the nature of the cations in solution. So, at 508C the sorption of Fe(III) containing ions on AV-17 at the same values as in condition I (see Fig. 2) is greater from solutions containing KNO 3 compared to solutions containing NaNO 3 (Figs. 3 and 4). According to Ref. [12] the K-jarosite is more stable than the Na- or H-jarosite. We suggest that on sorption of Fe(III) containing ions from Fe 2 (SO 4 ) 3 solution on strongly basic anion exchangers R 4 N[Fe 3 (SO 4 ) 2 (OH) 6 ] and H[Fe 3 (SO 4 ) 2 (OH) 6 ] are formed in the resin phase. When KNO 3 is present in Fe 2 (SO 4 ) 3 solutions in contact with resins at 508C processes (6) and (7) could be set up: R 4 N[Fe 3 (SO 4 ) 2 (OH) 6 ] 1 K 1 ⇔K[Fe 3 (SO 4 ) 2 (OH) 6 ] 1 R 4 N 1

4. The sorption isotherms The experimentally obtained isotherms of Fe(III) ions sorption on anion exchangers AV-17 and Varion-AD were analysed using the Freundlich, Temkin and Langmuir equations. The Freundlich Eq. (8) is an empirical one: S5a 3C

1/n

(8)

where S is value of sorption (mg Fe / g), C is the concentration of Fe (III) ions in solution (mg Fe / ml) and a and n are constants the physical and chemical nature of which was not established. The application of the Freundlich equation suggests that sorption energy exponentially decreases on completion of the sorptional centres of a sorbent. The Temkin equation suggests a linear decrease of sorption energy as the degree of completion of the sorptional centres of a sorbent is increased. Ignoring very low and very large values of concentration, the Temkin equation is shown as Eq. (9) [15,16]:

(6)

S 5 a 1 b ln C

(7)

where a and b are constants, of which the nature is not definite, and S and C are as in Eq. (8).

H[Fe 3 (SO 4 ) 2 (OH) 6 ] 1 K 1 ⇔K[Fe 3 (SO 4 ) 2 (OH) 6 ] 1 H 1

The existence of processes (6) and (7) was examined. A sample of AV-17 was placed in contact with 2310 22 M Fe 2 (SO 4 ) 3 solution containing K 2 SO 4 (I50.2) for 24 h at 508C. After washing with distilled water and desorption of Fe(III) ions using 1.5 M HCl solution, 2.34 mg K 1 / g was found in the filtrate. Process (7) also was individually investigated. A 0.5-g sample of AV-17 sorbed Fe (III) ions from 500 ml of 2310 22 M Fe 2 (SO 4 ) 3 solution at 508C. Then after filtration the polymer sample was placed in 50 ml of distilled water. At equilibrium the pH of the system was 2.05. After adding some KCl crystals, the pH of the system became 1.85.

(9)

V. Gutsanu et al. / Reactive & Functional Polymers 46 (2001) 203 – 211

The application of the Langmuir equation [Eq. (10)] suggests that the sorptional energy is constant and does not depend on the degree of occupation of the active centres of a sorbent: S 5 Sm 3 KC / 1 1 KC

(10)

where K and Sm are constants and S and C are as in Eq. (8). Unlike the constants a and b in the Freundlich and Temkin equations, the constants, K and Sm , in the Langmuir equation have a definite meaning. K is as an affinity index and reflects the sorptional mechanism or nature of sorptional centres and Sm is a quantitative expression of sorptional centres. The linearity of Eq. (10) is respected only at low soluion concentration. For sorption on the sorbents containing two energetically different kinds of sorptional centers, Langmiur proposed the following equation of the isotherm [Eq. (11)]:

209

S 5 (S 9m K9C / 1 1 K9C) 1 (S 99m K0C / 1 1 K0C) (11) where the constants K9, S 9m and K0, S 99m characterise the centres with more and less affinity, respectively. Eq. (11) is more effective than Eq. (10). It permits us to describe the sorption isotherm in a larger range of solution concentrations. Under finite conditions the sorption may take place only on centres with less affinity the high affinity centres being completed. In this case the Langmur equation [Eq. (12)] is applied: S 5 S0 9m 1 S0 9m K09C / 1 1 K09C

(12)

There are some methods to resolve Eqs. (8)– (12). Most efficient is the method of non-linear regressions [16–18]. Using this method we approximated the experimentally obtained sorption isotherms (Figs. 7 and 8), using Eqs. (8)– (12). The calculated parameters of these equa-

Fig. 7. Experimental (*) and calculated (—) isotherms of Fe(III) containing ions sorption on anion exchangers AV-17 and Varion-AD using the Freundlich (A, B) and Temkin (C, D) equations.

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Fig. 8. Experimental (*) and calculated (—) isotherms of Fe(III) containing ions sorption on anion exchangers AV-17 and Varion-AD using Langmuir Eq. (10) (A, B), (11) (C, D) and (12) (E, F).

tions are listed in Table 2. Analysis of the results (statistical analysis and approximation of the experimental and calculated isotherms) show that the Langmuir equations [Eqs. (10)–(12)] describe the Fe(III) ions sorption isotherms on the AV-17 and Varion-AD polymers more adequately than the Freundlich or Temkin equations. Eqs. (10)–(12) show practically the same affinity of the less active sorptional centres (K0

and K). These equations also describe identically the sorption capacities of polymers which are determined by the low affinity sorptional centres that result from the almost equal values of Sm and S 99m . The higher affinity sorptional centres (S 99m ) constitute only about 0.04–0.09% according to Eq. (12) or 0.03–0.04% according to Eq. (11). This means that practically all active centres (R 4 N 1 ) of polymers AV-17 and VarionAD are energetically the same. The data in

V. Gutsanu et al. / Reactive & Functional Polymers 46 (2001) 203 – 211

211

Table 2 Parameters of Freundlich, Temkin and Langmuir equations for Fe(III) containing ions sorption isotherms on anion exchangers AV-17 and Varion-AD AV-17

Varion-AD

Freundlich Eq. (8) a n 14.72 1.788

a 24.06

n 1.731

Temkin Eq. (9) a 16.09

a 27.14

b 13.05

K (ml / mg) 0.907

Sm (mg / g) 56.24

K9 (ml / mg) 9.030

S m9 (mg / g) 0.018

K0 (ml / mg) 0.901

S0 9m (mg / g) 0.051

K0 (ml / mg) 0.902

S 99m (mg / g) 56.340

b 7.11

Langmuir Eq. (10) K (ml / mg) Sm (mg / g) 0.926 32.91 Langmuir Eq. (11) K9 (ml / mg) S 9m (mg / g) 9.010 0.013

K0 (ml / mg) 0.921

Langmuir Eq. (12) S0 9m (mg / g) K0 (ml / mg) 0.015 0.923

S 99m (mg / g) 32.920

S m99 (mg / g) 32.970

Table 2 show that amount of high so as of low affinity sorbtional centres is more in Varion-AD than in AV-17. References [1] V.L. Gutsanu, V.A. Gafiichuk, Zh. Fiz. Khim. 60 (1986) 1824. [2] V.L. Gutsanu, V.A. Gafiichuk, Khim. Tekhnolog. Vody. 11 (1989) 584. [3] V.L. Gutsanu, C.I. Turta, V.A. Gafiichuk, V.N. Shofransky, Zh. Fiz. Khim. 62 (1988) 2415. [4] A.Z. Hrynkiewicz, I. Kubisz, D.S. Kulgawczuk, J. Inorg. Nucl. Chem. 27 (1965) 2513. [5] V. Gutsanu, C. Luca, V. Neagu et al., React. Funct. Polymer 40 (1999) 123. [6] J.P. Suzdalev, Gamma Resonance Spectroscopy of Proteins and Model Compounds, Nauka, Moscow, 1988.

S m99 (mg / g) 56.520

[7] A.A. Lurie, Sorbents and Chromatographic Carriers, Nauka, Moscow, 1972. [8] A.S. Plachinda, E.F. Macarov, S.I. Alexeeva, E.V. Egorov, J. Inorg. Nucl. Chem. 38 (1976) 859. [9] I.F. Fishtic, I.I. Vataman, Thermodynamics of the Metallic Ions Hydrolysis, Shtiintsa, Chisinau, 1988. [10] V.V. Plachevsky (Ed.), Complex-forming in Redox Systems, Vol. III, Dushanbe, 1976. [11] G.N. Belozerskii, M.V. Baikov, V.V. Boldyrev et al., Kinetika i Kataliz 15 (1974) 929. [12] D.K. Arkhipenko, E.T. Devyatkina, N.A. Palchik, Crystallochemical Particularities of Synthetic Jarosites, Nauka, Novosibirsk, 1987. [13] M. Ohyabu, Y. Ujihira, J. Inorg. Nucl. Chem. 43 (1981) 1948. [14] G. Dogaru, V. Gutsanu, Zh. Fiz. Khim. 54 (1980) 2318. [15] B.W. Bache, E.G. Williams, J. Soil Sci. 22 (1971) 289. [16] I.A. Mead, J. Soil Res. 19 (1981) 333. [17] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. [18] I.C.R. Holford, Aust. J. Soil Res. 20 (1982) 233.