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Acid–base properties of ion exchangers. III. Anion exchangers on the basis of polyacrylonitrile fiber
Reactive & Functional Polymers 63 (2005) 27–34
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Acid–base properties of ion exchangers. III. Anion exchangers on the basis of polyacrylonitrile fiber A.A. Shunkevich a, Z.I. Akulich a, G.V. Mediak a, V.S. Soldatov a
a,b,*
Institute of Physical Organic Chemistry of the Belarus Academy of Sciences, 13 Surganov Street, Minsk 220072, Belarus b Technical University of Lublin, 40-618 Nadbystrzycka Street, Lublin, Poland Received 23 June 2004; received in revised form 31 January 2005; accepted 3 February 2005 Available online 16 March 2005
Abstract The anion exchange fibers on the basis of industrially produced polyacrylonitrile fiber NITRON with different strong and weak base groups have been described in the paper. Their acid–base properties were characterized by the acidity parameters calculated from the potentiometric titration curves. The method of computer analysis of the titration curves described elsewhere was used. It was established that the ion exchange fibers are polyfunctional and contain at least four types of functional groups. Their capacities and the acidity parameters have been calculated. The fibers have full ion exchange capacity about 3.5 meq/g. The capacity according to the strong base quaternary aminogroups (pK @ 2) is usually about 2.5 meq/g. The water uptake, stability in hydrogen peroxide solutions, strong acids and alkalis of different concentrations at the ambient temperature and 100 C have been studied. The fibers do not loose significantly their anion exchange capacities and mechanical properties except for the most extreme regimes of their treatment. At the same time they accumulate carboxylic acid groups in their structure due to hydrolysis of the residual nitrile groups in the fiber. 2005 Elsevier B.V. All rights reserved. Keywords: Ion exchangers; Ion exchange fibers; Fibrous ion exchangers; Potentiometric titration
1. Introduction Polyacrylonitrile (PAN) fibers are convenient staring materials for syntheses of ion exchangers [1,2]. Some fibrous anion exchangers synthesized *
Corresponding author. Tel./fax: +375 17 284 2338. E-mail address: [email protected] (V.S. Soldatov).
on the basis of industrially produced PAN fibers have been describers in Refs. [1–9]. These materials are industrially produced under trademarks FIBAN [6] and VION [4,10]. Several varieties of ion exchangers of this type containing weak base amino groups and carboxylic acid groups have found an application in air purification from acid gases [11,12]. They also can be used as efficient
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.02.002
28
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
sorbents of cationic and anionic species of the heavy, transition and noble metals from aqueous media [13]. Similar to the other fibrous ion exchangers their main advantage compared to the granular resins is a unique combination of extremely fast rate of sorption and easy permeability of their filtering layers for liquids. Nevertheless, it is not clear whether potential advantages of anion exchange fibers of this type can be realized in practice due to the lack of information of their acid– base properties, chemical stability and mechanical properties. It is the aim of this paper to present such data on a family of newly synthesized ion exchange fibers on the PAN matrix containing quaternary ammonium groups of different chemical structure.
2. Experimental The matrix polymer was industrially produced PAN fiber Nitron D (Novopolotsk Industrial Association POLYMIR, Belarus). It is obtained by copolymerization of acrylonitrile, methylmethacrylate and itaconic acid (92.5, 6.3 and 1.2 mass%, respectively). The ion exchangers were prepared from the staple fibers with the diameter 18 ± 0.5 lm and length 60–80 mm. The ion exchange fibers described in this paper were obtained by functionalization of the Nitron fibers by substituting the nitrile groups with anion exchange groups of different nature. Due to the hydrolysis of the nitrile groups in the processes of their syntheses (a side reaction in our case) the resulting ion exchange fibers contained some amount of carboxylic acid groups. They also contained residual nitrile group. Before investigation of their properties the ion exchange fibers were treated in the same manner as the conventional ion exchange resins of a similar chemical type. They were washed out alternatively three times by 0.5 M NaOH and HCl, converted into a desired ionic form and dried to air-dry state under the ambient conditions. The samples used for the potentiometric titration were converted into OH form by treatment with carbonate free NaOH solution and stored with precautions against sorption of CO2 from the
atmosphere. At such treatment the carboxylic acid groups converted in Na+ form but in the following washing out with large amount of water to the neutral pH the sodium form was hydrolyzed and the finally the fiber contained a small amount of –COOH groups (H+/OH form). The fibers were primarily characterized by the values of their anion and cation exchange capacities. For their determination the specimen of the fiber 0.2 g in mixed H+/Cl–OH form was placed in the contact with excessive amount of 0.1 M HCl or NaOH (20 ml of 0.1 solution) and the concentration of the alkali or acid in the contacting solution was determined by titration of the aliquot after more than 6 h contact time. The exchange capacities were calculated from the consumption or the acid or base. The acid is consumed for neutralization of the functional groups in OH form and in the free amine form. The alkali is consumed for neutralization of the carboxylic acid groups and substituting of Cl with OH in the anionic functional groups. Special experiments on substitution of Cl with 1 M KNO3 in the column conditions have given identical results. This means that the Cl under condition of our experiment was practically completely substituted by OH. In order to determine separately the amount of the functional groups in Cl form another aliquot part of the alkaline solution was mercurimetrically titrated for chloride ions by Hg(NO3)2. In some cases, the anion exchange capacity according to strong and weak base groups was determined by Shtamberg and Yurachka [14] method. The water uptake was measured by using centrifugation method. The wet samples with mass about 0.2 g were centrifuged for 15 min at centrifugal acceleration 1431g. The chemical stability of the ion exchange fibers was characterized by changing their anionic and cationic exchange capacities and water uptake after keeping the samples in different aggressive media at temperatures 20 and 100 C; 0.5 and 5.0 M HCl and NaOH as well as 10% H2O2 aqueous solutions were used for these tests. The regime of treatment and characteristics of the fibers after the treatment are presented in Table 2. The mechanical properties of the fibers before and after the same treatment were characterized
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
by the force (F gram) and elongation (%) at rupture of the filaments. These properties were measured by device UMIV-3 (Russian production) used in textile industry. Ten filaments were taken for the measurement in each case and the mean value was calculated. The tensile strength (r kg/mm2) was calculated from the F value and diameter of the fiber measured by the optical microscope with micrometric scale. The results of these tests are given in Tables 3 and 4. The thermal stability of the ion exchange fibers was studied by thermo-gravimetric method using derivatograph of system Paulik, Paulik and Erdey (Hungary). The measurements were done in air with the rate of heating 5 C per minute. The potentiometric titration of the ion exchange fibers was performed by using the one-sample method in the variant described in detail in Ref. [15]. The samples of ion exchange fiber with the mass about 0.2 g where placed in 1 M NaCl or KCl and the titrant (0.1 M NaOH + 1 M NaCl or 0.1 M KCl + 1 M KCl) was added by weight portions with the 20-min interval. This time interval was sufficient for achieving the equilibrium state in each point. This was proved by coincidence of the curves of the forward and backward titration of the same sample of the fiber. The pH was registered by laboratory digital pH meter OP-211/1 with combined electrode OP-0808P (Hungary production). The other details of the experiment are given in Ref. [15]. The computer analysis of titration curves was done according to procedure described in Ref. [16]. The acid–base properties of the fibers were characterized by the parameters of the acidity [17] calculated from the curves of their potentiometric titration.
29
3. Results and discussion The ion exchange fibers described are new materials developed for use in aqueous and gaseous media. Knowledge of their chemical, mechanical and thermal stability as well as the acid–base properties is necessary to outline possible fields and conditions of their application. The chemical structure of the predominant functional groups and main properties of the fibers under investigation are given in Table 1. Predominant exchange groups in the synthesized ion exchangers are anion exchange ones. The carboxylic acid groups are formed in the fibers due to uncontrolled hydrolysis of the nitrile groups in the reaction media Hþ ;OH
R-CN þ 2H2 O ! R-COOH þ NH3 This process occurs at the contact of the ion exchangers containing nitrile groups with aqueous media and it has to be considered at their application for water treatment. The nitrile hydrolysis leads to changing properties of the ion exchanger and to contamination of the water with ammonia. In the present studies, we evaluated the scale of these undesired processes. It has been established that the rate of hydrolysis is approximately the same for all studied ion exchange fibers. Here, we present the data for fibers A-5 and A-6. The latter has found practical application in air and water purification and deserves a special attention. No accumulation of carboxylic acid groups at a prolong contact (1–2 months) of these fibers with aqueous solutions with pH in the range 3–9 was detected by measuring of the cation exchange capacity. Nevertheless, a more sensitive test for
Table 1 The main properties of the fibrous ion exchangers FIBAN type
Predominant functional groups
E meq/g, strong base
E meq/g, weak base
E meq/g, weak acid
W, water uptake g H2O/g fiber
A-5 A-6 A-7 A-8 A-9
–N(CH3)2 –(C3H5O)(CH3)2 N+Cl–N(CH3)2 –(C2H5O)(CH3)2 N+Cl–N(CH3)2 –(CH3)3 N+Cl–N(CH3)2 –(C6H5CH2)(CH3)2 N+Cl–N(CH3)2
– 2.36 ± 0.02 2.54 ± 0.02 2.63 ± 0.02 2.65 ± 0.02
4.01 ± 0.04 0.53 ± 0.04 0.73 ± 0.04 0.68 ± 0.04 0.57 ± 0.04
0.54 ± 0.02 0.27 ± 0.02 0.27 ± 0.02 0.35 ± 0.02 0.78 ± 0.02
0.91 ± 0.05 0.76 ± 0.05 2.08 ± 0.05 0.49 ± 0.05 1.61 ± 0.05
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A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
Table 2 Effect of aggressive media onto properties of the ion exchange fibers FIBAN type
Chemicals and conditions of treatment
E ± 0.02 meq/g, strong base
E ± 0.04 meq/g weak base
E ± 0.04 meq/ g weak acid
W ± 0.05 g H2O/g fiber
K, mol H2O/equiv
A-5
0.5 M HCl, 0.5 h, 20 C 0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
0 0
3.99 3.83
0.32 0.62
1.14 1.18
14.7 14.7
0 0 0 0 0 0
3.84 3.77 3.95 2.07 3.66 3.73
1.05 1.60 3.77 7.78 0.92 0.63
1.18 1.31 1.07 0.65 1.18 1.66
13.5 13.5 7.7 3.7 14.3 21.2
2.04 1.81
0.82 0.85
0.52 0.82
0.76 0.96
12.6 15.3
1.26 0.53 0.32 0.25 1.62 1.68
1.56 2.12 2.17 1.37 0.94 0.80
– 1.13 – 4.67 – –
1.09 1.17 1.42 1.25 0.96 1.08
– 17.1 – 11.1 – –
A-6
0.5 M HCl, 0.5 h, 20 C 0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
ammonia in the contacting solutions indicated that this process occurred. The samples, carefully washed out from the chemicals used in the syntheses, released small amounts of ammonia into the aqueous solution with the ionic composition simi-
lar to the conventional drinking water. The concentration of NH3 in the water passed through the filtering bed of the FIBAN A-6 fiber under conditions similar to those in the free flow filters used for water purification at home conditions
Table 3 Mechanical properties of the ion exchange fibers after their treatment by aggressive chemicals Sample
Chemicals and conditions of treatment
d, lm
F, g
r, kg/mm2
e, %
Nitron D
–
17.9 ± 0.5
7.8 ± 1.1
30.8 ± 4.1
22.9 ± 4.3
FIBAN A-5
0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
28.7 ± 1.5 30.0 ± 1.7 30.9 ± 2.1 35.0 ± 1.3
5.2 ± 0.7 8.4 ± 1.2 6.0 ± 0.7 8.7 ± 1.2 4.6 ± 0.9 6.2 ± 0.3 5.0 ± 1.1 4.8 ± 0.9 The fiber loses integrity 3.4 ± 1.4 5.2 ± 1.5 4.4 ± 0.9 6.2 ± 1.7
22.1 ± 2.1 35.5 ± 3.1 37.8 ± 3.1 43.1 ± 2.0
0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
32.3 ± 1.9 28.7 ± 1.7 30.3 ± 2.1 34.2 ± 2.1
5.5 ± 0.7 7.0 ± 1.0 4.8 ± 0.8 7.7 ± 1.8 6.2 ± 1.4 8.5 ± 0.8 5.0 ± 1.0 4.8 ± 0.9 The fiber loses integrity 5.6 ± 1.5 7.1 ± 2.0 6.0 ± 0.7 8.7 ± 1.2
30.0 ± 4.4 34.1 ± 9.7 26.4 ± 3.2 41.6 ± 4.6
FIBAN A-6
30.5 ± 1.7 30.6 ± 1.8
31.9 ± 1.0 32.0 ± 1.2
25.0 ± 6.7 36.9 ± 4.6
30.4 ± 5.1 35.5 ± 3.1
Confidence interval for mathematical expectation of measured quantities were calculated with the reliability b = 0.9 under the number of measurements 10 P n P 7.
0.2 0.2 10.0 10.2 1.3 1.1 0.5 0.2 0 0.2 0.2 11.8 10.6 10.4 8.8 8.6 1.8 0.7 0.8 0.7 0.5 2.0 1.0 1.0 0.2 0.2 9.0 4.9 4.8 4.9 5.1 1.3 0.65 0.95 0.3 0.25 0 0.9 0.8 0 1.0 2.6 1.8 1.6 1.6 1.7 A-5 A-6 A-7 A-8 A-9
4.7 2.2 2.3 3.0 1.5
E2 ± 0.1 meq/g Dpk1 ± 0.2 pK1 ± 0.1 E1 ± 0.1, meq/g FIBAN type
Table 4 The acidity parameters of the fibrous anion exchangers
pK2 ± 0.2
Dpk2 ± 0.2
E3 ± 0.2 meq/g
pK3 ± 0.3
Dpk3 ± 0.2
E4 meq/g
pK4 ± 0.3
Dpk4 ± 0.2
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
31
(bed depth 90 mm, flow rate 2.1 cm/min) stabilizes at the level of about 0.2 mg/l. This is below the permissible level for potable water (0.5 mg/l). In a more severe test the fiber was kept in water at the mass ratio of the water to the fiber equal 10 for 72 h at room temperature. The concentration of NH3 in the water in this case reached 0.96 mg/l. This means that after prolong storing in the contact with water the filters intent to use for potable water conditioning should be rinsed with fresh water. It follows from these results that the synthesized ion exchange fibers are sufficiently stable and suitable for water treatment under ambient conditions. The effect of different aggressive media on the fibers has also been studied. Its result is illustrated in Table 2. The capacities given in the table are calculated per gram of the sample after treatment. It is seen from the table that fiber FIBAN A-5 well preserves the anion exchange capacity after treatment with the strong acid, alkali and oxidant, except for some loss of the capacity in 5 M NaOH at its prolong treatment at 100 C. At the same time it gains cation exchange capacity due to accumulation of carboxylic acid groups without a substantial changing of its swelling. Fiber FIBAN A-6, containing mainly strong base groups, looses them in the strong acid and, especially, in the alkali. The capacity on the weak base groups remains almost constant except for the wittingly unacceptable case of a prolong treatment of the fiber with 5 M NaOH at 100 C. Table 3 illustrates the influence of treatment of the fibers with aggressive media on their mechanical properties. The table shows that the fibers loose their mechanical properties with increasing the hydrolysis degree. Nevertheless, they do not loose their integrity except for the hardiest regime of their treatment (5 h treatment with 5 M NaOH at 100 C). The thermal stability of the anion exchange fibers is characterized by Fig. 1. It is seen that heating of the fibers up to temperature 150 C is accompanied by only one minimum on the differential thermo-gravimetric curves corresponding to the loss of mass below 20%. This corresponds to removal of the hydration water. The figure shows
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A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
(a)
(b)
200 250
630
340
610 310
100 280
100
T, ºC 100
200
300
400
T, ºC 500
600
700
800
0
0
0
20
20
40
40
m, %
m, %
0
60
200
300
400
500
600
700
800
600
700
800
60
80
80
100
100
(c)
100
(d) 100 680 110
350
350 300
300
T, ºC 100
200
300
400
T, ºC 500
600
700
800
0
0
0
20
20
40
40
m, %
m, %
0
60
100
200
300
400
500
60
80
80
100
100
(e) 100
400
T, ºC 0
100
200
300 400
500 600
700 800 900
0
m, %
20 40 60 80 100
Fig. 1. Differential and integral thermogravimetric curves of the FIBAN anion exchangers in Cl form: (a) A-5; (b) A-6; (c) A-7; (d) A-8; (e) A-9. Temperature rise rate 5 C/min.
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
that the anion exchangers in Cl form can resist at least short-time heating up to 150 C. The further increase in the temperature leads to complicated changes in the structure of the fibers whose nature has not been identified. The acid–base properties of the ion exchange fibers were characterized by the acidity parameters calculated from the potentiometric titration curves presented in Fig. 2. The neutralization process of the fibers appeared fast and 20-min interval between the additions of the consecutive titrant portions was sufficient for the attainment of the equilibrium. The titration curves contain one clearly expressed inflection point; in some cases they have another weak inflection. Nevertheless, the computer analysis shows that they contain at least four types of anion exchange groups because four sets
33
of the acidity parameters is required for their accurate description. Those are the exchange capacity according to a given type of the group i, Ei; the thermodynamic constant of ion exchange OH– An (in our case An = Cl) pKi at the fixed concentration of the supporting electrolyte; Dpki equal to the difference of the equilibrium coefficient of the OH–An exchange at the complete and zeroth degree of neutralization of the anion exchanger with the acid. In more detail the physical meaning of these quantities was described in Refs. [15,16]. Ion exchange fiber FIBAN A-5 was obtained by aminolysis of NITRON and according to the expected scheme of chemical reactions in the synthesis should contain ternary aminogroups and amidogroups. The latter have very weak basicity and probably may not be determined by standard
pH
pH
11 10 9 8 7 6 5 4 3 2
11
A-5
A-6
9 7 5 3
1
2
3
4
6 g
5
1
pH
2
3
4
5
6
7 g
pH
11
11
A-7
9
9
7
7
5
5
3
3
1
2
3
4
5
A-8
7 g
6
1
2
3
4
5
6 g
pH
13 11
A-9
9 7 5 3 1
2
3
4
5
6 g
Fig. 2. Titration curves of ion exchangers FIBAN by HCl in 1 M KCl (A-5, A-6), in 1 M NaCl (A-7, A-8, A-9). The points are experimental data, the lines are calculated as described in Ref. [16] with parameters given in Table 4.
34
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
methods mentioned above. It follows from the computer analysis of the titration curves that this fiber is polyfunctional and contains at least four type of base groups the strongest of which is amino groups with pK = 4.5, Dpki = 2.0. Ion exchange fibers FIBAN A-6–A-9 were obtained by quaternization of the ternary aminogroup of FIBAN A-5 with different quaterizing agents. They contain 1–3 meq/g of quaternary amino groups with different radicals. These groups have pK values significantly higher than that for the strong base resins of the type I and II (about 1 and 0.5, respectively [16]). In all the cases, the strong base fibers contain in a smaller amounts three other types of weak base groups with pK about 5, 9 and 11. The first value probably relates to residual ternary aminogroups, the others are not identified.
4. Conclusion The fibrous anion exchangers on the base polyacrylonitrile obtained by quaternization of the fiber with ternary aminogroups with different agents are polyfunctional materials containing strong base groups with pK 2 and at least three types of weak base groups. They are chemically and mechanically stable in aqueous media with pH = 1–12 and in hydrogen peroxide. In chloride form they allow heating in the air at 100 C. The fibers do not loose their anion exchange capacity in strong acid or strong base media but gain cation exchange capacity due to accumulation in their structure of the carboxylic acid groups at the ex-
pense of hydrolysis of residual nitrile groups in the polyacrylonitrile fibers. References [1] H. Egawa, J. Chem. Soc. Jpn. 68 (1965) 1304. [2] L.A. Wolf, A.I. Meos, Fibers of Specific Functions (in Russian), Khimiya, Moscow, 1971. [3] L.A. Wolf (Ed.), Fibers with Specific Properties (in Russian), Khimiya, Moscow, 1980. [4] M.P. Zverev, Chemisorption Fibers (in Russian), Khimiya, Moscow, 1981. [5] F. Vernon, T. Shah, React. Polym. 1 (1983) 301. [6] V.S. Soldatov, A.A. Shunkevich, G.I. Sergeev, React. Polym. 7 (1988) 159. [7] T. Kato, T. Kago, K. Kusakake, S. Morooka, H. Egawa, J. Chem. Eng. Jpn. 23 (1990) 744. [8] B.W. Zhang, K. Ficher, D. Bieniek, A. Kettrup, React. Polym. 24 (1994) 49. [9] Y. Lu, C. Wu, W.P. Lin, L.Y. Tang, H.M. Zeng, J. Appl. Polym. Sci. 53 (1994) 1461. [10] M.P. Zverev, Chem. Fibers 3 (1989) 32 (in Russian). [11] V.S. Soldatov, I.S. Elinson, A.A. Shunkevich, in: L. Pawlowski et al. (Eds.), Chemistry for the Protection of the Environment, Plenum Press, New York, 1986, p. 369. [12] V.S. Soldatov, I.S. Elinson, A.A. Shunkevich, L. Pawlowski, H. Wasag, in: L. Pawlowski et al. (Eds.), Chemistry for the Protection of the Environment. No. 2, Plenum Press, New York, 1996, p. 55. [13] V.S. Soldatov, A.A. Shunkevich, H. Wasag, L. Pawlowski, M. Pawlowska, in: L. Pawlowski et al. (Eds.), Environmental Engineering Studies, Plenum Publishers, New York, 2004, p. 153. [14] Yu. Shtamberg, F. Yurachka, J. Appl. Chem. 35 (1962) 2295 (in Russian). [15] V.S. Soldatov, Z.I. Sosinovich, T.A. Korshunova, T.V. Mironova, React. Funct. Polym. 58 (2004) 3. [16] V.S. Soldatov, Z.I. Sosinovich, T.V. Mironova, React. Fuct. Polym. 58 (2004) 13. [17] V.S. Soldatov, React. Funct. Polym. 2/3 (38) (1998) 73.
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Acid–base properties of ion exchangers. III. Anion exchangers on the basis of polyacrylonitrile fiber A.A. Shunkevich a, Z.I. Akulich a, G.V. Mediak a, V.S. Soldatov a
a,b,*
Institute of Physical Organic Chemistry of the Belarus Academy of Sciences, 13 Surganov Street, Minsk 220072, Belarus b Technical University of Lublin, 40-618 Nadbystrzycka Street, Lublin, Poland Received 23 June 2004; received in revised form 31 January 2005; accepted 3 February 2005 Available online 16 March 2005
Abstract The anion exchange fibers on the basis of industrially produced polyacrylonitrile fiber NITRON with different strong and weak base groups have been described in the paper. Their acid–base properties were characterized by the acidity parameters calculated from the potentiometric titration curves. The method of computer analysis of the titration curves described elsewhere was used. It was established that the ion exchange fibers are polyfunctional and contain at least four types of functional groups. Their capacities and the acidity parameters have been calculated. The fibers have full ion exchange capacity about 3.5 meq/g. The capacity according to the strong base quaternary aminogroups (pK @ 2) is usually about 2.5 meq/g. The water uptake, stability in hydrogen peroxide solutions, strong acids and alkalis of different concentrations at the ambient temperature and 100 C have been studied. The fibers do not loose significantly their anion exchange capacities and mechanical properties except for the most extreme regimes of their treatment. At the same time they accumulate carboxylic acid groups in their structure due to hydrolysis of the residual nitrile groups in the fiber. 2005 Elsevier B.V. All rights reserved. Keywords: Ion exchangers; Ion exchange fibers; Fibrous ion exchangers; Potentiometric titration
1. Introduction Polyacrylonitrile (PAN) fibers are convenient staring materials for syntheses of ion exchangers [1,2]. Some fibrous anion exchangers synthesized *
Corresponding author. Tel./fax: +375 17 284 2338. E-mail address: [email protected] (V.S. Soldatov).
on the basis of industrially produced PAN fibers have been describers in Refs. [1–9]. These materials are industrially produced under trademarks FIBAN [6] and VION [4,10]. Several varieties of ion exchangers of this type containing weak base amino groups and carboxylic acid groups have found an application in air purification from acid gases [11,12]. They also can be used as efficient
1381-5148/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.02.002
28
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
sorbents of cationic and anionic species of the heavy, transition and noble metals from aqueous media [13]. Similar to the other fibrous ion exchangers their main advantage compared to the granular resins is a unique combination of extremely fast rate of sorption and easy permeability of their filtering layers for liquids. Nevertheless, it is not clear whether potential advantages of anion exchange fibers of this type can be realized in practice due to the lack of information of their acid– base properties, chemical stability and mechanical properties. It is the aim of this paper to present such data on a family of newly synthesized ion exchange fibers on the PAN matrix containing quaternary ammonium groups of different chemical structure.
2. Experimental The matrix polymer was industrially produced PAN fiber Nitron D (Novopolotsk Industrial Association POLYMIR, Belarus). It is obtained by copolymerization of acrylonitrile, methylmethacrylate and itaconic acid (92.5, 6.3 and 1.2 mass%, respectively). The ion exchangers were prepared from the staple fibers with the diameter 18 ± 0.5 lm and length 60–80 mm. The ion exchange fibers described in this paper were obtained by functionalization of the Nitron fibers by substituting the nitrile groups with anion exchange groups of different nature. Due to the hydrolysis of the nitrile groups in the processes of their syntheses (a side reaction in our case) the resulting ion exchange fibers contained some amount of carboxylic acid groups. They also contained residual nitrile group. Before investigation of their properties the ion exchange fibers were treated in the same manner as the conventional ion exchange resins of a similar chemical type. They were washed out alternatively three times by 0.5 M NaOH and HCl, converted into a desired ionic form and dried to air-dry state under the ambient conditions. The samples used for the potentiometric titration were converted into OH form by treatment with carbonate free NaOH solution and stored with precautions against sorption of CO2 from the
atmosphere. At such treatment the carboxylic acid groups converted in Na+ form but in the following washing out with large amount of water to the neutral pH the sodium form was hydrolyzed and the finally the fiber contained a small amount of –COOH groups (H+/OH form). The fibers were primarily characterized by the values of their anion and cation exchange capacities. For their determination the specimen of the fiber 0.2 g in mixed H+/Cl–OH form was placed in the contact with excessive amount of 0.1 M HCl or NaOH (20 ml of 0.1 solution) and the concentration of the alkali or acid in the contacting solution was determined by titration of the aliquot after more than 6 h contact time. The exchange capacities were calculated from the consumption or the acid or base. The acid is consumed for neutralization of the functional groups in OH form and in the free amine form. The alkali is consumed for neutralization of the carboxylic acid groups and substituting of Cl with OH in the anionic functional groups. Special experiments on substitution of Cl with 1 M KNO3 in the column conditions have given identical results. This means that the Cl under condition of our experiment was practically completely substituted by OH. In order to determine separately the amount of the functional groups in Cl form another aliquot part of the alkaline solution was mercurimetrically titrated for chloride ions by Hg(NO3)2. In some cases, the anion exchange capacity according to strong and weak base groups was determined by Shtamberg and Yurachka [14] method. The water uptake was measured by using centrifugation method. The wet samples with mass about 0.2 g were centrifuged for 15 min at centrifugal acceleration 1431g. The chemical stability of the ion exchange fibers was characterized by changing their anionic and cationic exchange capacities and water uptake after keeping the samples in different aggressive media at temperatures 20 and 100 C; 0.5 and 5.0 M HCl and NaOH as well as 10% H2O2 aqueous solutions were used for these tests. The regime of treatment and characteristics of the fibers after the treatment are presented in Table 2. The mechanical properties of the fibers before and after the same treatment were characterized
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
by the force (F gram) and elongation (%) at rupture of the filaments. These properties were measured by device UMIV-3 (Russian production) used in textile industry. Ten filaments were taken for the measurement in each case and the mean value was calculated. The tensile strength (r kg/mm2) was calculated from the F value and diameter of the fiber measured by the optical microscope with micrometric scale. The results of these tests are given in Tables 3 and 4. The thermal stability of the ion exchange fibers was studied by thermo-gravimetric method using derivatograph of system Paulik, Paulik and Erdey (Hungary). The measurements were done in air with the rate of heating 5 C per minute. The potentiometric titration of the ion exchange fibers was performed by using the one-sample method in the variant described in detail in Ref. [15]. The samples of ion exchange fiber with the mass about 0.2 g where placed in 1 M NaCl or KCl and the titrant (0.1 M NaOH + 1 M NaCl or 0.1 M KCl + 1 M KCl) was added by weight portions with the 20-min interval. This time interval was sufficient for achieving the equilibrium state in each point. This was proved by coincidence of the curves of the forward and backward titration of the same sample of the fiber. The pH was registered by laboratory digital pH meter OP-211/1 with combined electrode OP-0808P (Hungary production). The other details of the experiment are given in Ref. [15]. The computer analysis of titration curves was done according to procedure described in Ref. [16]. The acid–base properties of the fibers were characterized by the parameters of the acidity [17] calculated from the curves of their potentiometric titration.
29
3. Results and discussion The ion exchange fibers described are new materials developed for use in aqueous and gaseous media. Knowledge of their chemical, mechanical and thermal stability as well as the acid–base properties is necessary to outline possible fields and conditions of their application. The chemical structure of the predominant functional groups and main properties of the fibers under investigation are given in Table 1. Predominant exchange groups in the synthesized ion exchangers are anion exchange ones. The carboxylic acid groups are formed in the fibers due to uncontrolled hydrolysis of the nitrile groups in the reaction media Hþ ;OH
R-CN þ 2H2 O ! R-COOH þ NH3 This process occurs at the contact of the ion exchangers containing nitrile groups with aqueous media and it has to be considered at their application for water treatment. The nitrile hydrolysis leads to changing properties of the ion exchanger and to contamination of the water with ammonia. In the present studies, we evaluated the scale of these undesired processes. It has been established that the rate of hydrolysis is approximately the same for all studied ion exchange fibers. Here, we present the data for fibers A-5 and A-6. The latter has found practical application in air and water purification and deserves a special attention. No accumulation of carboxylic acid groups at a prolong contact (1–2 months) of these fibers with aqueous solutions with pH in the range 3–9 was detected by measuring of the cation exchange capacity. Nevertheless, a more sensitive test for
Table 1 The main properties of the fibrous ion exchangers FIBAN type
Predominant functional groups
E meq/g, strong base
E meq/g, weak base
E meq/g, weak acid
W, water uptake g H2O/g fiber
A-5 A-6 A-7 A-8 A-9
–N(CH3)2 –(C3H5O)(CH3)2 N+Cl–N(CH3)2 –(C2H5O)(CH3)2 N+Cl–N(CH3)2 –(CH3)3 N+Cl–N(CH3)2 –(C6H5CH2)(CH3)2 N+Cl–N(CH3)2
– 2.36 ± 0.02 2.54 ± 0.02 2.63 ± 0.02 2.65 ± 0.02
4.01 ± 0.04 0.53 ± 0.04 0.73 ± 0.04 0.68 ± 0.04 0.57 ± 0.04
0.54 ± 0.02 0.27 ± 0.02 0.27 ± 0.02 0.35 ± 0.02 0.78 ± 0.02
0.91 ± 0.05 0.76 ± 0.05 2.08 ± 0.05 0.49 ± 0.05 1.61 ± 0.05
30
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
Table 2 Effect of aggressive media onto properties of the ion exchange fibers FIBAN type
Chemicals and conditions of treatment
E ± 0.02 meq/g, strong base
E ± 0.04 meq/g weak base
E ± 0.04 meq/ g weak acid
W ± 0.05 g H2O/g fiber
K, mol H2O/equiv
A-5
0.5 M HCl, 0.5 h, 20 C 0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
0 0
3.99 3.83
0.32 0.62
1.14 1.18
14.7 14.7
0 0 0 0 0 0
3.84 3.77 3.95 2.07 3.66 3.73
1.05 1.60 3.77 7.78 0.92 0.63
1.18 1.31 1.07 0.65 1.18 1.66
13.5 13.5 7.7 3.7 14.3 21.2
2.04 1.81
0.82 0.85
0.52 0.82
0.76 0.96
12.6 15.3
1.26 0.53 0.32 0.25 1.62 1.68
1.56 2.12 2.17 1.37 0.94 0.80
– 1.13 – 4.67 – –
1.09 1.17 1.42 1.25 0.96 1.08
– 17.1 – 11.1 – –
A-6
0.5 M HCl, 0.5 h, 20 C 0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
ammonia in the contacting solutions indicated that this process occurred. The samples, carefully washed out from the chemicals used in the syntheses, released small amounts of ammonia into the aqueous solution with the ionic composition simi-
lar to the conventional drinking water. The concentration of NH3 in the water passed through the filtering bed of the FIBAN A-6 fiber under conditions similar to those in the free flow filters used for water purification at home conditions
Table 3 Mechanical properties of the ion exchange fibers after their treatment by aggressive chemicals Sample
Chemicals and conditions of treatment
d, lm
F, g
r, kg/mm2
e, %
Nitron D
–
17.9 ± 0.5
7.8 ± 1.1
30.8 ± 4.1
22.9 ± 4.3
FIBAN A-5
0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
28.7 ± 1.5 30.0 ± 1.7 30.9 ± 2.1 35.0 ± 1.3
5.2 ± 0.7 8.4 ± 1.2 6.0 ± 0.7 8.7 ± 1.2 4.6 ± 0.9 6.2 ± 0.3 5.0 ± 1.1 4.8 ± 0.9 The fiber loses integrity 3.4 ± 1.4 5.2 ± 1.5 4.4 ± 0.9 6.2 ± 1.7
22.1 ± 2.1 35.5 ± 3.1 37.8 ± 3.1 43.1 ± 2.0
0.5 M HCl–0.5 M NaOH, 3 times, 5 h, 20 C 5 M HCl, 0.5 h, 100 C 5 M HCl, 5.0 h, 100 C 5 M NaOH, 0.5 h, 100 C 5 M NaOH, 5.0 h, 100 C 10% H2O2, 48 h, 20 C 10% H2O2, 96 h, 20 C
32.3 ± 1.9 28.7 ± 1.7 30.3 ± 2.1 34.2 ± 2.1
5.5 ± 0.7 7.0 ± 1.0 4.8 ± 0.8 7.7 ± 1.8 6.2 ± 1.4 8.5 ± 0.8 5.0 ± 1.0 4.8 ± 0.9 The fiber loses integrity 5.6 ± 1.5 7.1 ± 2.0 6.0 ± 0.7 8.7 ± 1.2
30.0 ± 4.4 34.1 ± 9.7 26.4 ± 3.2 41.6 ± 4.6
FIBAN A-6
30.5 ± 1.7 30.6 ± 1.8
31.9 ± 1.0 32.0 ± 1.2
25.0 ± 6.7 36.9 ± 4.6
30.4 ± 5.1 35.5 ± 3.1
Confidence interval for mathematical expectation of measured quantities were calculated with the reliability b = 0.9 under the number of measurements 10 P n P 7.
0.2 0.2 10.0 10.2 1.3 1.1 0.5 0.2 0 0.2 0.2 11.8 10.6 10.4 8.8 8.6 1.8 0.7 0.8 0.7 0.5 2.0 1.0 1.0 0.2 0.2 9.0 4.9 4.8 4.9 5.1 1.3 0.65 0.95 0.3 0.25 0 0.9 0.8 0 1.0 2.6 1.8 1.6 1.6 1.7 A-5 A-6 A-7 A-8 A-9
4.7 2.2 2.3 3.0 1.5
E2 ± 0.1 meq/g Dpk1 ± 0.2 pK1 ± 0.1 E1 ± 0.1, meq/g FIBAN type
Table 4 The acidity parameters of the fibrous anion exchangers
pK2 ± 0.2
Dpk2 ± 0.2
E3 ± 0.2 meq/g
pK3 ± 0.3
Dpk3 ± 0.2
E4 meq/g
pK4 ± 0.3
Dpk4 ± 0.2
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
31
(bed depth 90 mm, flow rate 2.1 cm/min) stabilizes at the level of about 0.2 mg/l. This is below the permissible level for potable water (0.5 mg/l). In a more severe test the fiber was kept in water at the mass ratio of the water to the fiber equal 10 for 72 h at room temperature. The concentration of NH3 in the water in this case reached 0.96 mg/l. This means that after prolong storing in the contact with water the filters intent to use for potable water conditioning should be rinsed with fresh water. It follows from these results that the synthesized ion exchange fibers are sufficiently stable and suitable for water treatment under ambient conditions. The effect of different aggressive media on the fibers has also been studied. Its result is illustrated in Table 2. The capacities given in the table are calculated per gram of the sample after treatment. It is seen from the table that fiber FIBAN A-5 well preserves the anion exchange capacity after treatment with the strong acid, alkali and oxidant, except for some loss of the capacity in 5 M NaOH at its prolong treatment at 100 C. At the same time it gains cation exchange capacity due to accumulation of carboxylic acid groups without a substantial changing of its swelling. Fiber FIBAN A-6, containing mainly strong base groups, looses them in the strong acid and, especially, in the alkali. The capacity on the weak base groups remains almost constant except for the wittingly unacceptable case of a prolong treatment of the fiber with 5 M NaOH at 100 C. Table 3 illustrates the influence of treatment of the fibers with aggressive media on their mechanical properties. The table shows that the fibers loose their mechanical properties with increasing the hydrolysis degree. Nevertheless, they do not loose their integrity except for the hardiest regime of their treatment (5 h treatment with 5 M NaOH at 100 C). The thermal stability of the anion exchange fibers is characterized by Fig. 1. It is seen that heating of the fibers up to temperature 150 C is accompanied by only one minimum on the differential thermo-gravimetric curves corresponding to the loss of mass below 20%. This corresponds to removal of the hydration water. The figure shows
32
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
(a)
(b)
200 250
630
340
610 310
100 280
100
T, ºC 100
200
300
400
T, ºC 500
600
700
800
0
0
0
20
20
40
40
m, %
m, %
0
60
200
300
400
500
600
700
800
600
700
800
60
80
80
100
100
(c)
100
(d) 100 680 110
350
350 300
300
T, ºC 100
200
300
400
T, ºC 500
600
700
800
0
0
0
20
20
40
40
m, %
m, %
0
60
100
200
300
400
500
60
80
80
100
100
(e) 100
400
T, ºC 0
100
200
300 400
500 600
700 800 900
0
m, %
20 40 60 80 100
Fig. 1. Differential and integral thermogravimetric curves of the FIBAN anion exchangers in Cl form: (a) A-5; (b) A-6; (c) A-7; (d) A-8; (e) A-9. Temperature rise rate 5 C/min.
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
that the anion exchangers in Cl form can resist at least short-time heating up to 150 C. The further increase in the temperature leads to complicated changes in the structure of the fibers whose nature has not been identified. The acid–base properties of the ion exchange fibers were characterized by the acidity parameters calculated from the potentiometric titration curves presented in Fig. 2. The neutralization process of the fibers appeared fast and 20-min interval between the additions of the consecutive titrant portions was sufficient for the attainment of the equilibrium. The titration curves contain one clearly expressed inflection point; in some cases they have another weak inflection. Nevertheless, the computer analysis shows that they contain at least four types of anion exchange groups because four sets
33
of the acidity parameters is required for their accurate description. Those are the exchange capacity according to a given type of the group i, Ei; the thermodynamic constant of ion exchange OH– An (in our case An = Cl) pKi at the fixed concentration of the supporting electrolyte; Dpki equal to the difference of the equilibrium coefficient of the OH–An exchange at the complete and zeroth degree of neutralization of the anion exchanger with the acid. In more detail the physical meaning of these quantities was described in Refs. [15,16]. Ion exchange fiber FIBAN A-5 was obtained by aminolysis of NITRON and according to the expected scheme of chemical reactions in the synthesis should contain ternary aminogroups and amidogroups. The latter have very weak basicity and probably may not be determined by standard
pH
pH
11 10 9 8 7 6 5 4 3 2
11
A-5
A-6
9 7 5 3
1
2
3
4
6 g
5
1
pH
2
3
4
5
6
7 g
pH
11
11
A-7
9
9
7
7
5
5
3
3
1
2
3
4
5
A-8
7 g
6
1
2
3
4
5
6 g
pH
13 11
A-9
9 7 5 3 1
2
3
4
5
6 g
Fig. 2. Titration curves of ion exchangers FIBAN by HCl in 1 M KCl (A-5, A-6), in 1 M NaCl (A-7, A-8, A-9). The points are experimental data, the lines are calculated as described in Ref. [16] with parameters given in Table 4.
34
A.A. Shunkevich et al. / Reactive & Functional Polymers 63 (2005) 27–34
methods mentioned above. It follows from the computer analysis of the titration curves that this fiber is polyfunctional and contains at least four type of base groups the strongest of which is amino groups with pK = 4.5, Dpki = 2.0. Ion exchange fibers FIBAN A-6–A-9 were obtained by quaternization of the ternary aminogroup of FIBAN A-5 with different quaterizing agents. They contain 1–3 meq/g of quaternary amino groups with different radicals. These groups have pK values significantly higher than that for the strong base resins of the type I and II (about 1 and 0.5, respectively [16]). In all the cases, the strong base fibers contain in a smaller amounts three other types of weak base groups with pK about 5, 9 and 11. The first value probably relates to residual ternary aminogroups, the others are not identified.
4. Conclusion The fibrous anion exchangers on the base polyacrylonitrile obtained by quaternization of the fiber with ternary aminogroups with different agents are polyfunctional materials containing strong base groups with pK 2 and at least three types of weak base groups. They are chemically and mechanically stable in aqueous media with pH = 1–12 and in hydrogen peroxide. In chloride form they allow heating in the air at 100 C. The fibers do not loose their anion exchange capacity in strong acid or strong base media but gain cation exchange capacity due to accumulation in their structure of the carboxylic acid groups at the ex-
pense of hydrolysis of residual nitrile groups in the polyacrylonitrile fibers. References [1] H. Egawa, J. Chem. Soc. Jpn. 68 (1965) 1304. [2] L.A. Wolf, A.I. Meos, Fibers of Specific Functions (in Russian), Khimiya, Moscow, 1971. [3] L.A. Wolf (Ed.), Fibers with Specific Properties (in Russian), Khimiya, Moscow, 1980. [4] M.P. Zverev, Chemisorption Fibers (in Russian), Khimiya, Moscow, 1981. [5] F. Vernon, T. Shah, React. Polym. 1 (1983) 301. [6] V.S. Soldatov, A.A. Shunkevich, G.I. Sergeev, React. Polym. 7 (1988) 159. [7] T. Kato, T. Kago, K. Kusakake, S. Morooka, H. Egawa, J. Chem. Eng. Jpn. 23 (1990) 744. [8] B.W. Zhang, K. Ficher, D. Bieniek, A. Kettrup, React. Polym. 24 (1994) 49. [9] Y. Lu, C. Wu, W.P. Lin, L.Y. Tang, H.M. Zeng, J. Appl. Polym. Sci. 53 (1994) 1461. [10] M.P. Zverev, Chem. Fibers 3 (1989) 32 (in Russian). [11] V.S. Soldatov, I.S. Elinson, A.A. Shunkevich, in: L. Pawlowski et al. (Eds.), Chemistry for the Protection of the Environment, Plenum Press, New York, 1986, p. 369. [12] V.S. Soldatov, I.S. Elinson, A.A. Shunkevich, L. Pawlowski, H. Wasag, in: L. Pawlowski et al. (Eds.), Chemistry for the Protection of the Environment. No. 2, Plenum Press, New York, 1996, p. 55. [13] V.S. Soldatov, A.A. Shunkevich, H. Wasag, L. Pawlowski, M. Pawlowska, in: L. Pawlowski et al. (Eds.), Environmental Engineering Studies, Plenum Publishers, New York, 2004, p. 153. [14] Yu. Shtamberg, F. Yurachka, J. Appl. Chem. 35 (1962) 2295 (in Russian). [15] V.S. Soldatov, Z.I. Sosinovich, T.A. Korshunova, T.V. Mironova, React. Funct. Polym. 58 (2004) 3. [16] V.S. Soldatov, Z.I. Sosinovich, T.V. Mironova, React. Fuct. Polym. 58 (2004) 13. [17] V.S. Soldatov, React. Funct. Polym. 2/3 (38) (1998) 73.