Sorption of bivalent ions by a fibrous chelating ion exchanger and the structure of sorption complexes

Reactive & Functional Polymers 71 (2011) 49–61

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Sorption of bivalent ions by a fibrous chelating ion exchanger and the structure of sorption complexes V.S. Soldatov a,b, V.M. Zelenkovskii a,⇑, L.A. Orlovskaya a a b

Institute of Physical Organic Chemistry Belarus National Academy of Sciences, 13 Surganov Str., Minsk 220072, Belarus Lublin University of Technology, Ul. Nadbysrzycka 40B, 20-618 Lublin, Poland

a r t i c l e

i n f o

Article history: Received 28 May 2010 Received in revised form 3 September 2010 Accepted 7 November 2010 Available online 11 November 2010 Keywords: Ion exchangers Selective ion exchange Chelating ion exchangers Fibrous ion exchangers Ion exchange fibers Heavy metal ions Quantum chemical calculations Ion-polymeric complexes

a b s t r a c t Acid–base properties, kinetics and equilibrium sorption of Ca2+, Cu2+, Pb2+, Cd2+, Ni2+ and Mn2+ ions on fibrous chelating ion exchanger FIBAN X-1 containing iminodiacetic group have been studied. The structural and electronic characteristics of ion-polymeric complexes have been obtained by non-empirical quantum chemical calculations. The rate of sorption processes was extremely high; the half-process time of the sorption process was equal to 24 s. The isotherms of ion sorption are described by the Langmuir equation what allowed deriving equation for dependence of the distribution coefficient on the ion concentration and accurate extrapolation of these value to zeros concentration. The concentration range the Henry’s law applicability was determined theoretically and proved experimentally. The sorption selectivity series in a neutral medium was Cu > Ni > Pb > Cd > Mn > Ca. The same series follows the pH at which the sorption begins. Complete extraction of all heavy metal ions occurs at pH > 4. The quantum chemical calculations have shown that the selectivity of sorption is determined by the number of coordination bonds between the cation and fixed anion, the length of the bonds and degree of their covalence. The way of combining of these factors is individual for each specific cation and can be revealed by quantum chemical calculations. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Chelating ion exchangers with iminodiacetic (IDA)1 group on styrene–divinylbenzene matrixes are the most spread ion selective sorbents. Industrially produced granular sorbents of this type (Dowex A-1, Chelex 100, Wofatit MC 50, ANKB-50 and the number of others) have been used in analytical chemistry for pre-concentration of trace amounts of heavy metals from aqueous media preceded spectroscopic measuring of their concentration for at least 25 years [1]. Nevertheless the works on further improvement of this technique are still continued (see for example Refs. [2–7]. Properties of these sorbents were extensively investigated and described in numerous original papers, reviews and monographs (e.g. [8–16]). Application of ion exchangers with IDA groups in water purification from the heavy metal ions have been also described in numerous publications [17–22]. The main problem in applications of granular chelate sorbents is a poor kinetics of sorption processes. It has been shown in a number of publications that filtering layers of fibrous ion ⇑ Corresponding author. Tel.: +375 17 284 2373; fax: +375 17 284 1679. E-mail address: [email protected] (V.M. Zelenkovskii). Naming AN(CH2COOH)2 group iminodiacetic is incorrect because it is not a derivative of imine (R0 R00 C@NAR). One of the correct naming is aminodiacetic group. Nevertheless we are using term iminodiacetic (IDA) to avoid discrepancy with the other papers on ion exchange and complex chemistry. 1

1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.11.003

exchangers with the same or similar functional groups have significantly higher sorption rate and a better performance in dynamic processes [23–29]. The rate of sorption on the fibrous ion exchangers is comparable to that for the microspheric resins of the same chemical type. That is due to the shortness of diffusion path of the absorbing ions in the fibers whose radius is usually 5–20 lm. The advantage of fibers compared to the microspheric resins is a low hydrodynamic resistance. This allows using shallow filtering beds (2–5 cm) with a fast linear eluent velocity (5–15 cm/min) and pressure drop <20 kPa. The half-process time of sorption on the chelate ion exchange fibers is usually less than 1 min compared to several hours on the conventional chelate ion exchangers [22]. We suppose that chelate fibrous ion exchangers can find analytical applications as highly permeable, efficient and inexpensive packed beds for pre-concentration of the target ions using only the natural gravity force for passing the water probe through the pre-concentration column [30]. Information of kinetic properties, sorption equilibria and structure of the sorption complexes of fibrous sorbents with IDA groups is rather scarce and scattered in different literature sources. The latter is known only in too general terms: their geometry, number and type of ligands, length and degree of covalence of the chemical bonds in the complex were not quantitatively characterized. In this publication we describe properties of fibrous ion exchanger FIBAN X-1 obtained by chemical modification of industrial polyacrylonitrile fiber [31].

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The acid base properties of this ion exchanger, kinetics of sorption and sorption equilibria of Ca2+, Cu2+, Pb2+, Cd2+, Ni2+ and Mn2+ ions in dependence of pH and ion concentration have been studied. The structure of the ion-polymer complexes have been calculated using ab initio quantum chemical calculations and compared with the selectivity of the ion sorption. 2. Experimental Ion exchanger: Fibrous ion exchanger FIBAN X-1 was produced at the experimental plant of the Institute of Physical Organic Chemistry Belarus National Academy of Sciences (Minsk, Rep. of Belarus). The industrial polyacrylonitrile fiber Nitron C (copolymer of acrylonitrile 92.5%, methylacrylate 6.3% and itaconic acid 1.2%) with titer 0.33 tex has been used as a starting material for preparation of the sorbent. The fibers are uniform in thickness. The cross section of this fiber has a complicated shape (‘‘kidney shape’’) [32]. The parent fiber was treated consequently by ethylenediamine in aqueous medium and monochloroacetic acid in alkali media [31]. It contains IDA and carboxylic acid groups fixed on the polyacrylic matrix. Its main properties are presented in Table 1. Chemicals: The commercial Merk standard solutions and all salts of investigated metals, high purity nitric acid of analytical grade salts in deionized water (18.2 MX) were used in the experiments. Measurements of the concentrations of the heavy metal ions was done by atomic absorption spectrometry with plasma and electrothermal atomization with Zeeman correction of non-selective absorption on ASS VARIO 6 and HITACHI Z-2000 instruments. Concentration of calcium was determined on ICP ARL 3410+ spectrometer. Potentiometric titration: One-sample titration method in variant described in detail in Ref. [33] has been used for obtaining the titration curves. The same sample of ion exchanger 0.4788 g, in H form cut in the pieces less than 5 mm long was placed in contact with 30.0 mL of 1.00 M KCl and 2.40 mL of 1.00 M HCl + 1.00 M KCl was added to the solution. This was done in order to protonize the nitrogen atom of IDA group and possible residual amino groups nonreacted with monocloroacetic acid. The forward titration was done by the solution of KOH 1.94 M + KCl 1.00 M. The back titration – with 2.00 M HCl + 1.00 M KCl. The titrant was added by portions 0.1–0.3 g by a transfer pipette weight before and after the addition with precision 0.2 mg in 20 min interval. The solution density was measured in a separate experiment. The suspension was vigorously stirred by magnetic stirrer. The combined glass electrode was permanently immersed in the solution and the stirrer was not stopped at the addition of each new portion of the titrant after recording the pH. The high concentration of the titrants was taken to avoid dilution of the solution in the titration process. The titration cell was coved with a cup having apertures opened for a short time for introducing the titrant and taking out the sample for analysis. The pH was measured on HANNA HI 9321 ionomer with combined electrode HI 1131B. The sorption of bivalent ions in dependence of the solution pH was also studied in the course of the potentiometric titration. It Table 1 The main properties of ion exchanger FIBAN X-1 in H form. Matrix

Predominant functional groups

Acrylic

CH2COOH N CH2COOH COOH

Cationic capacity (meq/g)

Water uptake (gH2O/g)

3.74

0.60

was carried out in the presence of the free acid in order to measure the metal sorption from acid solutions. In order to obtain simultaneously the titration curve and values of the metal sorption at different pH 0.5 mL of 0.01 M solution of the bivalent ion was added to the initial solution contacting with the ion exchanger. The amount of bivalent ions added was about 3% from the capacity of the ion exchanger in the titration cell. Portions of the solution 0.25 mL were taken out from the titration cell before adding the next portion of titrant and analyzed for the cation. The volume of the solution in the titration cell remained practically constant because removed for analyses solution and that of the added titrant were equal. The amount of absorbed ions was calculated from the difference in initial and current concentration of it in the solution. From these data the fraction of ions extracted from the solution after the contact with ion exchanger, R have been calculated. Kinetic experiment: The rate of sorption of the bivalent ions was studied under conditions of constant volume. The sample of fiber (H-form, 50 mg), cut into pieces about 5 mm long, was placed into a beaker with 100 mL of the acetate buffer with pH = 6.00 and intensively stirred with magnetic stirrer at 20 °C. Than 0.1 mL of 0.01 M solution of a salt of bivalent cation was introduced by a high precision sampler and the time of the beginning the process was fixed. The stirrer rate was 300 r/min; its doubling did not affect the sorption rate. Aliquots of the solution (0.5 mL) were taken out in 0.5, 1, 2, 5, 10 and 20 min and analyzed for the bivalent ion. The amount of absorbed ion was calculated from the difference in concentration of the ion before and after the contact with ion exchanger. Sorption experiments: The sorption of bivalent ions was studied at batch conditions from acetate buffer with pH = 6.00 and concentration of Na+ 0.19 mol L1. The samples of ion exchanger with preliminary determined contents of the dry matter weight with precision 0.2 mg and mass 50 mg were placed in flasks and the solutions of nitrates of the bivalent cations in the buffer were added. The total volume of the solution was 50.0 mL. The flasks were shaken during 1 h. In separate experiments it was established that this time guaranties the sorption equilibrium achievement. The concentration of metal ions was determined in the aliquots by atomic absorption spectrometry as described above. The sorption value was calculated from the difference in the Me concentration before and after the contact with the sorbent. These data were used for obtaining the sorption isotherms. The distribution coefficient was calculated from equation

D¼

gM ; CM

ð1Þ

where gM is the content of absorbing ion in the sorbent in mill moles per gram of dry sorbent in H form; CM – its concentration in the equilibrium solution in mol L1. Quantum chemical calculations: We applied program pack GAMESS [34] The calculations were done by non-empiric method ROHF SCF MO LCAO using the minimal basis set MINI [35]. The full gradient optimizing of all geometric parameters was performed for achieving the minimum of the full energy of the calculated systems. All discussed further structural and electronic characteristics of the complexes are related to the minima of the potential energy. In more detail the method of quantum chemical modeling is described in Ref. [36]. 3. Results and discussion 3.1. Acid properties of the ion exchanger and sorption of the bivalent ions as a function of the solution pH Acid–base propertied of the ion exchanger were characterized by its titration curve given in Fig. 1. These data were used to

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V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

ka ¼ x  C H =ðð1  xÞ  C K Þ;

ð2Þ +

where x is the equivalent fraction of K in the ion exchanger (degree of neutralization) and Ci is the concentration of the relative cation in the solution. Ka is the equilibrium constant [38]:

lg ka ¼

Z

1.0

Degree of extraction

calculate the amount of each kind of ionizable groups in the fiber Ei (meq/g) and their acidity parameters pKa and Dpka. These determinations have been done using the model and a computer program described in Refs. [33,37]. The model allowed calculation of the capacity of polyfunctional ion exchanger according to each type of protonizable groups and the parameters of their acidity. Their meanings are as follows: The ka is an equilibrium coefficient of ion exchange of H+ with the cation of titrant (in our case K+)

0.8 0.6 Cu Pb Ni Cd Mn

0.4 0.2 0.0 1

2

3

4

lg ka dx

ð3Þ

0

5

6

7

pH

1

Fig. 2. Degree of extraction of bivalent ions from the solution as a function of pH.

In approximation of linear dependence of lg ka = f(x) accepted in the model

pK a ¼ pka ðx ¼ 1=2Þ

ð4Þ

Dpk ¼ pkx¼1  pkx¼0

ð5Þ

It is seen from Fig. 1 that the results of forward and back titration are identical indicating that the equilibrium in the titration system was achieved. The ion exchanger contains four types of distinguishable acid groups probably relating consequently to the first step of protonizing of the IDA group, free carboxylic acid group, second step of IDA group ionization and very weak acid groups, probably carboxylic groups bound into intermolecular associate with the nitrogen of the IDA groups. The pK0 = 0, Dpk0 = 0 correspond to titration of the free HCl introduced in the solution. The pK1 corresponding to the first step of ion exchange H+–K+ on the IDA group is in good agreement with the intrinsic dissociation constant obtained by Krasner and Marinsky [39]. Their value, pK = 2.7, is related to x = 0. The value pkx=0 calculated from the pK1 = 3.1 and Dpk1 = 1.0 is 2.6. The fraction of the heavy metal ions removed from the solution in dependence of pH was calculated from the results of titration of the ion exchanger in the presence of ions of the heavy metal and the concentration of the metal in the solution at different pH (Fig. 2). The minimal pH values at which the bivalent ions were completely removed from the solution are given in Table 2. It is

pH

Table 2 Minimal solution pH corresponding to 100% extraction of bivalent ions from the contacting solution. Ion

Cu2+

Ni2+

Cd2+

Pb2+

Mn2

pH

2.0

3.0

5.0

5.0

7.0

also seen that the sorption started already from acid media. This means that ion exchanger FIBAN X-1 is suitable for concentration of ions from any kind of natural waters. This property has been used for recommendation of this sorbent as a pre-concentrator of ions of heavy metals in analytical practice. These data correlate well with experimental results in work [40]. 3.2. Rate of sorption The kinetic curves in coordinates (g/gmax)–t1/2 were obtained for the all ions. Here (g/gmax) is a fraction of the equilibrium sorption, t is a time from the beginning of the process. An example of such dependences is given in Fig. 3. The kinetic curves for all studied ions were approximately the same. The half-process time (t1/2) was 24 s, and the process completed in 5 min. At the degree of saturation of the sorbent with Cu2+ less than 80% the sorption process is described by the Weber and Morris equation [41]

11

g/g max

9 7

0.8

5

0.6

3

0.4

g/gmax 0.8 0.6

t1/2

0.4 0.2

1

2

3

4

5

6

7

8 g

Fig. 1. Potentiometric titration curve of ion exchanger FIBAN X-1. The points are experimental data for forward (j) and back (.) titration. The line is calculated using the following capacities and the acidity parameters: E0 = 5.0, pK0 = 0, Dpk0 = 0; E1 = 1.37, pK1 = 3.1, Dpk1 = 1.0; E2 = 1.1, pK2 = 5.0, Dpk2 = 1; E3 = 1.37, pK3 = 9.0, Dpk3 = 1; E4 = 1.1, pK4 = 10.0, Dpk4 = 1.0. The parameters with subscript ‘‘0’’ relate to titration of free HCl.

0.2 0.0 0

0.0

0

2

4

6

8

10

t1/2,seconds 5

10

15

20

25

30

t1/2,seconds Fig. 3. Dependence (g/gmax)–t1/2 for sorption of Cu2+.

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

ðg=g max Þ ¼ A  t1=2

the layer of this ion exchanger because in this case the loading of the sorbent with the target ion is only 1–2% of the total capacity. This sorbent can be used for concentration of trace amounts of the heavy metal ions from aqueous media in small cells with a short filtering layers and a high rate of the water flow at the field conditions for the following laboratory analyses.

ð6Þ 1/2

with constant A = 0.104 s . Independence of the process rate on the stirring speed of the suspension ion exchanger – solution proves that its rate is limited by intra-particle diffusion. This is in line with the results of work of Yoshioko and Shimamura obtained for fibrous composition ion exchanger Ionex with IDA groups [29]. The cylindrical fibers of this ion exchanger consist on mechanical mixture of polyethylene and cross-linked sulfonated polystyrene reinforced by strings of polyethylene (the ‘‘islands-in-the-sea’’ structure). The process of sorption of copper (II) was well described by the kinetic equation for isotope exchange by the ion exchanger with continuous regular structure and endless cylindrical particles.

t 1=2 ¼

0:065r 2 D

3.3. The sorption isotherms and distribution coefficients The practical interest for analytical purposes represents knowledge of the distribution coefficient at concentrations of the target ions in the solutions equal or below Maximal Admissible Concentration, i.e. <1 mg L1 for Cu2+, <0.03 for Pb2+, >0.0001 for Cd2+, <0.1 for Ni2+ and Mn2+. At these concentrations reliable direct determination of some of these ions by standard atomic absorption spectrometry is impossible. Using sorbent FIBAN X-1 it was possible to concentrate these ions from dilute solution and reliably determine their concentration after desorption with a small volume of nitric acid. Therefore determination of the distribution coefficients at concentrations tending to zero deserves practical interest. Their values can be determined by extrapolation of D from analytically accessible concentration if the extrapolation equation is known. We tested applicability for this purpose the Langmuir equation

ð7Þ

(a)

Content of lead g(Pb) mmol/g of resin

In our case the fiber has kidney shape cross section to which the concept of ‘‘radius’’ is not applicable and the diffusion equation have not been derived. Nevertheless as a palliative we used for semi-qualitative evaluation of the diffusion coefficient Eq. (7) with the ‘‘apparent radius’’ value 10l, equal to the shortest distance from the periphery to the fiber middle in place of the r. The value of ‘‘apparent diffusion coefficient’’ is 2.7  109 cm2 s1, i.e. of the same order of magnitude as that obtained in Ref. [33]. It follows from these data that several seconds contact time under dynamic conditions would be sufficient for complete extraction of the trace amounts of ions from the solution passed through

gM ¼

(b) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

-5

4.0x10

-5

8.0x10

-4

-4

1.2x10

1.6x10 3

KEC M ; 1 þ KC M

Content of lead g(Pb) mmol/g of resin

52

0.3

0.2

0.1

0.0 0.0

4.0x10

-6

8.0x10

-6

1.2x10

-5

3

С(Pb) solution concentration, mol/dm

С(Pb) solution concentration, mol/dm

(c)

ð8Þ

(d)

7

5

6

4

5 3

lgD

1/g

4 3

2

2 1 1 0 0.0

0 4.0x10

4

8.0x10

1/C

4

1.2x10

5

1.6x10

5

-10 -9

-8

-7

-6 -5 lgC

-4

-3

-2

Fig. 4. Sorption of Pb2+. Points are experimental data, the lines are calculated. a and b – dependences of sorption on the equilibrium concentration Pb2+ in the solution; the lines are calculated from the parameters of Langmuir equation (Table 3) found from linearized dependence 1/g = f (1/C). c dependence 1/g = f (1/C); the line is obtained by the least square fit (see the equation in column 2 of Table 3). d dependence lg D = f(lg C); the line is calculated from Eq. (10).

53

(a)

(b) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

-5

-5

-5

-5

2.0x10 4.0x10 6.0x10 8.0x10

Content of copper g(Cu) mmol/g of resin

Content of copper g(Cu) mmol/g of resin

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

3

(d)

14

0.2

0.1

0.0

0.0

-8

5.0x10

-7

1.0x10

-7

1.5x10

-7

2.0x10

12

6

10

5

8

4

6 4

3

7

lgD

1/g

0.3

С(Cu) solution concentration, mol/dm

С(Cu) solution concentration, mol/dm

(c)

0.4

3 2

2

1

0

0

7

1x10

7

2x10

7

3x10

7

4x10

0 -10

7

5x10

-9

-8

1/C

-7

-6

-5

-4

-3

lgC 2+

Fig. 5. Sorption of Cu . See caption to Fig. 4.

Table 3 Parameters of Langmuir equation and distribution coefficient an C = 0.

This equation in logarithmic form has two asymptotes:

Ion

Linearized Langmuir equation

DC M !0

E1 Mmole/ g

K

Ca2+ Cu2+ Pb2+ Cd2+ Ni2+ Mn2+

1/g = 8.0  104 1/g = 2.0  107 1/g = 3.0  105 1/g = 5.0  105 1/g = 5.0  106 1/g = 3.0  104

1.25  103 5.0  106 3.3  104 2.0  104 2.0  105 3.3  103

0.70 0.76 0.76 0.68 0.74 0.76

1.79  103 6.58  106 4.34  104 3.57  104 3.13  105 4.30  103

(1/C) + 1.42 (1/C) + 1.31 (1/C) + 1.33 (1/C) + 1.47 (1/C) + 1.36 (1/C) + 1.47

here E is a capacity of the sorbent to the target ion at the chosen solution pH; K is a constant of sorption. Determination of the applicability of this equation to description of our experimental data is illustrated by Figs. 4 and 5. Parameters E and K were found by linearization of dependence

1 1 1 1  ¼ þ ; g M E EK C M

ð9Þ

by the least square fit method. They are presented in Table 3. It appeared that the equation is applicable to the low concentration in all studied cased but significant deviation was observed for the Cu2+ and Mn2+ at higher concentrations. Using parameters E and K theoretical dependences of D on the CM were calculated from equation

D¼

EK : 1 þ KC M

ð10Þ

lg D ¼ lg E þ lg K; ðC M ! 0Þ

ð11Þ

lg D ¼ lg E  lg C M ðC M ! 1Þ

ð12Þ

Dependences lg D = f (lg CM) are presented in Figs. 4 and 5. The distribution coefficients tend to the constant values

Dc!0 ¼ E  K

ð13Þ

At concentrations lower than the point of intersection of the asymptotes (lg CM = lg K) dependences lg D = f(lg C) have a region with almost constant D which can be defined as a concentration range in which the Henry law is applicable with a good precision. For example distribution coefficient of Cu2+ at the point of the asymptotes intersection (C = 1.6  107 M) is 3.2  106; the limiting value DC=0 = 5.0  106. For Pb2+ (C = 1.6  105) these figures are respectively 3.0  104 and 3.3  104. Values of DC=0 are presented in Table 4. It is seen that the maximal sorption E found from Langmuir equation is almost the same for all ions and lower than the full ion exchange capacity. The full capacity can be realized only in the alkaline medium. In our conditions (pH = 6.00) only approximately a half of it can be realized, as it follows from the titration data in Fig. 1. Dependences of the distribution coefficient on the concentration of bivalent ions in the solution are given in Fig. 6. It is seen that the plateau at a low concentrations and a decrease at a higher concentrations is a common feature of the systems studied. Its nature will be discussed in the following section of the paper.

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V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

Table 4 Structural and electronic characteristics of ion-polymer complexes bivalent ions with IDA groups. Element

Effective charge on the cation

Total number of water molecules in the complex

Number of Me ligands

Number of Me bonds with water molecules

Number of Me bonds with the fixed anion

The bond

Length of the bond, nm

The bond order

The distribution coefficient Dc?0

Ca

1.805

9

6

4

2

1.664

8

6

4

2

Pb

1.094

8

5

3

2

Ni

1.473

5

4

1

3

Cu

1.606

5

6

2

4

Mn

1.940

5

6

5

1

0.227 0.233 0.224 0.227 0.245 0.225 0.181 0.182 0.194 0.186 0.190 0.210 0.256 0.196

– – 0.103 0.083 0.238 0.379 0.241 0.214 0.176 0.158 0.181 0.103 – 0.179

1.25  103

Cd

CaAO CaAO CdAO CdAO PbAO PbAO NiAO NiAO NiAN CuAO CuAO CuAN CuAN MnAO

lgD 7.0

2.0  104 3.3  104 2.0  105

5.0  106

3.30  103

3.4. Structure of the sorption complexes obtained by ab initio quantum chemical calculations

1

6.5 6.0 5.5

2

5.0 4.5 4.0

3 4 5

3.5

6

3.0 2.5 2.0 1.5 -11 -10

-9

-8

-7

-6

-5

-4

-3

-2

-1

lgC

Fig. 6. lg D as a function of logarithm of the equilibrium concentrations of metal ions in the buffer pH = 6.00. 1 – Cu2+, 2 – Ni2+, 3 – Pb2+, 4 – Cd2+, 5 – Mn2+, 6 – Ca2+.

Difference in the selectivity of ions can be understood if the structure of their complexes with the functional group of the sorbent is known. It is usually assumed that their structure is similar to that of their non-polymer analogues. The drawings of structures given in the literature (e.g. [42]) usually have an illustrative character and based on general chemical knowledge rather than on concrete studies of their specific structure. They do not specify location of the interacting ions, ligands and water molecules in the complex, the length and character of the bonds between them. The IDA group contains three potential ligands: two oxygen atoms of the carboxylic acid groups and the nitrogen atom and can form complexes with different number of ligands and different strength depending on the chemical nature of the cation, pH and amount of water molecules accessible for their hydration. Therefore for understanding their structure it is not sufficient to have

O O

H

H

H

H O

H2O

0.234

H2C

OH2

0.232 H O

0.231

H

0.233 0,236

H2C O H H

H2O H

O H

O

n -effective electrical charge n- bond length Fig. 7. The scheme of interaction of Ca2+ with IDA group of the sorbent.

OH2

55

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

in the forms of metal ions (Ca2+, Cd2+, Pb2+, Ni2+, Cu2+, Mn2+) and 12 water molecules. The calculated system contained 67 or 133 atoms. The following information has been obtained: coordinates of all atoms, the full energy of the system, electrical charges on the atoms and the bond orders (the quantity characterizing the degree of bond covalence). The 3D images of the complexes which can be drawn on the base of these data contain many elements and have a complex configuration difficult for understanding. Because of that we present the structural formulas of the complexes without the water molecules not interacting directly with the metal ions or atoms of the functional group. The solid lines in the drawings join the symbols of atoms if the distances between their centers exceed the sum of their radii less than by 25%. The bold lines denote covalent bonds between the atoms of functional group which practically does not change in the process of complex formation. The thin solid lines denote bonds formed between atoms in the process of

a simple drawing of their expected structure. Sometimes the complexes depictured identically (for example complexes of IDA with Ca2+ and Pb2+) may have stability constants differing by many orders. The concrete information of the geometry of the complexes and the nature of the bonds between interacting atoms can be obtained from quantum chemical calculations. The ion exchanger was modeled by molecular fragment containing a part of the hydrocarbon chain and one or two functional neighboring groups

H2C CH C

O CH2COOH

HN

(CH2)2

N CH2COOH

O O

H

H

H

H O

OH 2

H2O

0,241

H 2C

0,235 0,240 H H

O

0,236 OH 2

0,237

0,233 H 2C

H

OH 2

H 2O

O

O

O

H H

0,242

H

H

H

O

Fig. 8. The scheme of interaction of Ca2+ with the neighboring IDA group the sorbent.

O O

H

H

H

H

O H 2O

0.232

OH 2

0.231 H O

0.229

H

0.238 0.232 O

OH 2

H 2O

H H 2+

Fig. 9. The scheme of interaction of Ca

O

with IDA group of the sorbent in system containing two HCl per two neighboring IDA groups; Cl is not shown in the figure.

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V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

O O

H

H

H

H O

OH2

H2O

0.237 0.230

H H

0.232

O

OH2

0.233 0.238

H

H

H2O

O

O

OH2

H

O

H H

H

O

Fig. 10. The scheme of interaction of Ca2+ with the neighboring IDA group of the sorbent in the acid medium; see caption to Fig. 9.

complex formation. The dotted lines denote hydrogen bonds. The distances between atoms in nanometers, charges on atoms in e.u. and the order of bonds (in square brackets) are also given in the drawings. 3.4.1. Calcium Ca2+ is the main competitor of the heavy metal ions in the sorption processes. Usually it is present in the aqueous media in much

greater concentrations than the ions of heavy metals. In the system with two closely situated functional groups, with one of them Ca2+ forms the classic chelate complex (Fig. 7) almost symmetrically interacting with one oxygen atoms of each carboxylic group. It coordinates four water molecules of nine in the hydrated complex. The length of MeAO bonds for the carboxylic groups and water molecules are almost the same. The charge on Ca atom is close to the nominal one (1.8 e.u.). The order of CaAO bonds is less than

O O

H

H

H

H O

H2O

0.223 0.091

0.227 0.103

OH2

0.224 0.089

H O

0.229

H

0.234

0.083

0.083

0.224 0.086 H2O

O

H H

H

O H

O

Fig. 11. The scheme of interaction of Cd2+ with IDA group of the sorbent.

OH2

57

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

O

O H

H

H

H O

0.245 0.238 H H

H

H

O

0.267 0.156

0.227 0.323 0.225 0.379

0.231 0.279

OH2

H2O

O H H

H

O

O H

Fig. 12. The scheme of interaction of Pb2+ with IDA group of the sorbent.

0.06 what means the negligently low degree of their covalence. The nitrogen atom does not participate in the complex formation because the distance NACa is too large.

O H

H

O O H

H

0.181 0.241

OH2

0.194 0.176

0.181 0.211

0.182 0.214

O H H H

O

O H

Fig. 13. The scheme of interaction of Ni2+ with IDA group of the sorbent.

The structure of complex formed by the neighboring IDA groups (Fig. 8) is slightly different from the first one. It includes 11 water molecules. Almost symmetric charge distribution between the O atoms of IDA group bound to Ca2+ occurring in the first case becomes asymmetric for the neighboring group. Nevertheless the summary electrical charge in the both cases is exactly the same: 0.621  0.638 = 0.601  0.658 = 1.295 e.u. The mean distances between O atoms of carboxylic group and Ca2+ slightly increases (respectively 0.230 and 0.237 nm). That means that the energy of electrostatic interaction in of Ca2+ with two neighboring IDA groups is almost the same and the selectivity of sorption should not strongly depend on the degree of loading of the sorbent with Ca2+. This is in agreement with the data on the dependence of distribution coefficient on the concentration of Ca2+ in the solution (Fig. 6). It is to note that the length of one of the bonds in the neighboring group is 0.240 nm that is higher than the limit which is denoted in the figures as a coordinative bond. Nevertheless the coordination of this atom (somewhat weaker) also occurs. Similar calculations for the system containing two pairs of ions H+ and Cl (imitation of the acid medium) per two IDA groups revealed the following regularities (Figs. 9 and 10). In the both structures one of the O atoms of carboxylic groups forms covalent bond with the hydrogen atom. Ca2+ ion coordinates another atom of the same carboxylic group, but the energy of this coordinative bond should be lower than that with non-protonated carboxylic group. The loss of energy of interaction Ca2+AO is partially compensated by increase in the charge on the O atom of another carboxyl in the IDA group. In the neighboring protonated carboxyl O atom is not bound to Ca2+ indicating the decrease in the energy ob interaction Ca2+ with IDA groups in acid media, what is a commonly known phenomenon, We can conclude that the presence of free protons does not qualitatively change the geometry of these complexes but substantially weaken the bonds of Ca2+ with the IDA group. This agrees with the fact of decay of these complexes in strong acid media.

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V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

O H

H

O H

H O

OH 2

0.184 0.191

OH 2

0.196 0.190

0.183 0.187

0.179 0.265 H

O

H

H

H H

H

O

O

O

Fig. 14. The scheme of interaction of Ni2+ with the neighboring IDA group of the sorbent.

At the same time that explains rather high sorption of Ca2+ from moderately acid solutions. 3.4.2. Cadmium The structure of cadmium complex is similar to that for calcium one (Fig. 11). In spite of the geometrical similarity the electronic and energetic characteristics of these complexes are quite different. The effective electrical charge on Cd atom is lower (1.6 e.u. compared

to 1.8 e.u. for Ca); the CdAO bonds are shorter and have a noticeable fraction of covalence. The order of these bonds with oxygen atoms of water and carboxylic acid groups are practically the same. It seems that increasing interaction of Cd2+ with carboxylic acid groups is compensated by the stronger bounding with the water molecules. This agrees well with the concept of competitive interaction of the counterions with the functional groups of the sorbent and water molecules in the nearest surrounding of the cation [43].

O H

H

O O H

H

0.180 0.271

OH2

0.194 0.182

0.181 0.215

0.188 0.156

O

H Fig. 15. The scheme of interaction of Ni

2+

H H

O

with IDA group of the sorbent in the acid medium.

59

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

tive electrical charge on Pb atom may not be compared with the charges on the other metal atoms due to peculiarities of Mulliken analysis of the electron level population.

O H

H

O H

H

O

OH 2

0.183 0.203 0.196 0.182

3.4.4. Nickel Ni2+ cation coordinates four ligands (Fig. 13). They are O atoms of each carboxylic group, N atom of IDA group and one O atom of water molecule. The degree of covalence of all coordinative bonds is high what is expressed in their high order and a relatively low effective electrical charge on the Ni atom. In the neighboring complex (Fig. 14) the coordination sphere contains two water molecules, the nitrogen atom and O atom of one carboxylic group. One Ni2+AO bonds of carboxylic acid group becomes broken indicating the decrease in the selectivity of Ni2+ sorption with increasing loading of the sorbent with Ni2+. The data in Fig. 6 are compatible with the results of quantum chemical calculations. These results show that we observe the effect of neighbor influencing dependence of the sorption energy (decreasing of the selectivity sorption) on the degree of loading of the sorbent with entering ions. This purely polymeric phenomenon is absent in dilute solutions of the non-polymer analogues. All bonds H2O  Ni2+, ACOO  Ni2+ and R3N  Ni2+ have a high degree of covalence. Introduction the proton into the medium of the complex formation, imitating the acid medium (Fig. 15), does not qualitatively change geometry of the group to which the proton binds. But the bond >C@O  Ni2+ in this group is significantly longer and less covalent than the bond >CAO  Ni2+ with dissociated carboxylic group. This corresponds to the decrease of the selectivity Ni2+ with the pH decrease. It is seen from Fig. 16 that the neighboring group also have somewhat different configuration in the acid media. The counterion forms coordination bond with the oxygen atom of nondissociated carboxylic group. The structure of the complex is the same as that in the absence of HCl but the bonds with O and N atoms of IDA groups are weaker and less covalent. At the same time the bonds with the water molecules in the acid medium is stronger what is expressed in their higher bond orders (compare Figs. 14 and 16).

OH 2

0.182 0.210

0.183 0.200 H O

O H

H OH 2 Fig. 16. The scheme of interaction of Ni2+ with the neighboring IDA group of the sorbent in the acid medium. The system contains two HCl per two IDA groups.

3.4.3. Lead Geometry of the lead complex (Fig. 12) is also similar to those for Ca2+ and Cd2+ but the Pb2+AO bonds have a very high degree of covalence (the bond order with one of the carboxylic acid group reaches 0.379). It is interesting to note that Pb2+ cation coordinates only three water molecules but the bonds with them have a high degree of covalence. One of them bridges Pb2+ with nitrogen of the IDA group. Such bridging is absent in the neighboring complex. A high degree of covalence of Pb2+ bonds with carboxylic groups explains a high selectivity of its sorption. It is to note that the effec-

H

O C H2C

H O

-0.660

H2O -0.637 0.063

0.181 0.182

0.217

0,103 0,210

-0.398

+1.606 -0.673

0.128 0.194

H2O -0.735

0.158 0.190

H C C H2 H

H O

CH2

-0.495

O

OH2

H2C

Fig. 17. The scheme of interaction of Cu2+ with IDA group of the sorbent.

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V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61

4. Conclusion

O CH2 CH2

H2C

C

C N H

N

O

CH2

CH2 C

H

-0.598 O

H

0.196 0.179

OH2 0.202 0.096

0,192 0,184 H2O 0.194 0.165

0.193 0.140

+1.94 OH 0.199 H2O 0.140

Chelating fibrous ion exchanger on the acrylic matrix with iminodiacetic groups reveals extremely fast sorption rate toward ions of heavy metals. The half-process time of the sorption process is 24 s. The selectivity of sorption of heavy metal ions compared to calcium is high and strongly depends on the chemical nature of the ion. The selectivity series at pH = 6.00 is Cu > Ni > Pb > Cd > Mn > Ca. The same series follows the pH at which the sorption begins. Complete extraction of all heavy metal ions occurs at pH > 4. At the fixed pH = 6 the sorption isotherms are well described by the Langmuir equation. This equation was used to derive equation for dependence of the distribution coefficient on the absorbing ions concentration and reliably extrapolate these values to their zeros concentration. The sorbent is suitable for extracting the heavy metal ions from aqueous media containing calcium in concentrations usual for natural waters using short sorbent beds and high water flow rates. The quantum chemical calculation of structure of the sorption complexes allowed establishing connections between the selectivity of sorption with electronic and structural characteristics of the complexes. The bounding of ions by the functional group of the sorbent is controlled by combining the following factors: number of coordination bonds between the cation and the fixed anion; the length of the bonds; the degree of their covalence. The way of combining of these factors is individual for each specific cation and can be revealed by quantum chemical calculations.

Fig. 18. The scheme of interaction of Mn2+ with IDA group of the sorbent.

Acknowledgements 3.4.5. Copper The copper ion forms strong bonds with two oxygen atoms of carboxylic acid groups and the nitrogen atom of IDA group. The electrostatic component of interaction between these atoms is high because the bonds are short and the electrical charges on the interacting atoms are relatively high. In the addition, the degree of their covalence is high enough while it is slightly lower than that in the case of Ni2+. Apart from the interaction with IDA group Cu2+ ion strongly electrostatically interacts with the nitrogen atom of peptide bond between the IDA group and the polymer chain (depictured by the bold dotted line in Fig. 17). The distance between Cu and this N atom is substantially larger than the sum of their ionic radii (by 0.04 nm) but the charges on the atoms are high and the charge screening atoms are absent. Under these conditions the long distance interaction can be a substantial addition to the summary interaction of the copper ion with the ion exchanger. 3.4.6. Manganese Manganese ion forms a complex completely different in structure with the structures of all other metals (Fig. 18). The IDA group is transformed into seven-member heterocycle with which manganese atom has one bond via the oxygen atom of one of the carboxylic acid groups of the IDA system. The charge on the Mn atom is almost equal to the nominal one (1.94 e.u.), but the order of this bond is rather high (0.179). Probably only this factor is responsible for a relatively high sorption of this ion from dilute solutions. The bonds of Mn2+ with five water molecules have a high degree of covalence. Strong interaction of Mn2+ with water is a factor lowering the selectivity of sorption of this ion. The data on quantum chemical calculation of structures the sorption complexes are summarized in Table 4. It is seen that the factors favoring a high selectivity of sorption are: a short distance between the cation and the coordinated atoms of the functional groups; larger number to such bonds; higher degree of their covalence.

This work was partially supported by Belarus Republican Foundation for Fundamental Research (Grant X09-091).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

[27]

H.M. Kingston, I.L. Barnes, Anal. Chem. 50 (1978) 2064–2070. Y. Zhu, A. Itoh, H. Haraguchi, Bull. Chem. Soc. Jpn. 78 (2005) 107–115. K. Lee, M. Oshima, S. Motomizu, Analyst 127 (2002) 769–774. S. Mito, M. Ohata, N. Furuta, Bunseki Kagaku 52 (2003) 575–582. M.S. Jimenez, R. Velarte, J.R. Castillo, Spectrochim. Acta – Part B Atom. Spectrosc. 57 (2002) 391–402. I.S. Trujillo, E.V. Alonso, M.T.S. Cordero, Anal. Atom. Spectrom. 25 (2010) 1063– 1071. D. Rahmi, Y. Takasaki, Y. Zhu, et al., Talanta 81 (2010) 1438–1445. A.A. Zagarodni, Ion Exchange Materials: Properties and Applications, Elsevier, 2007. G.V. Myasoedova, S.B. Savin, CRC Crit. Rev. Anal. Chem. 17 (1987) 1–64. M.V. Dinu, E.S. Dragan, A.W. Trochimczuk, Desalination 249 (2009) 374–379. C. Noureddine, A. Lekhmici, M.S. Mubarak, J. Appl. Polym. Sci. 107 (2008) 1316–1319. A.A. Atia, A.M. Donia, K.Z. Elwakeel, Sep. Purif. Technol. 43 (2005) 43–48. L.-C. Lin, R.-S. Juang, Chem. Eng. J. 112 (2005) 211–218. A. Agraval, K.K. Sahu, J. Hazard. Mater. 133 (2006) 299–303. M.V. Dinu, E.S. Dragan, React. Funct. Polym. 68 (2008) 1345–1354. L. Harju, T. Krook, Talanta 42 (1995) 431–436. A. Deepatana, M. Valix, Desalination 107 (2008) 334–342. L.-C. Lin, J.-K. Li, R.-S. Juang, Desalination 225 (2008) 249–259. M.E. Malla, M.B. Alvarez, D.A. Batistoni, Talanta 57 (2002) 277–287. E. Korngold, S. Belfer, C. Urtizberea, Desalination 104 (1996) 197–201. Z. Zainol, M.J. Nicol, Hydrometallurgy 99 (2009) 175–180. V.M. Zelenkovskii, L.A. Orlovskaya, V.S. Soldatov, et al., Doklady NAN Belarusi 51 (2007) 83–88. V.S. Soldatov, A.A. Shunkevich, I.S. Elinson, et al., Desalination 124 (1999) 181– 192. K.H. Lee, Y. Muraoka, M. Oshima, S. Motomizu, Anal. Sci. 20 (2004) 183–187. P. Hashemi, A. Olin, Talanta 44 (1997) 1037–1053. L.A. Orlovskaya, V.S. Soldatov, L.M. Kremko, Applicability of sorbent FIBAN X-1 for preconcentration of heavy metal ions in the analysis of aqueous media, in: J. Ostrowski (Ed.), Problemy Inzynerii Srodoviska. Material XXII Miezdunar. Sympoz. im. Boleslawa Krzistofica AQUA-2002, Politechnica Warsawska, Plock, 2002, p. 40. L.A. Orlovskaya, Sorption preconcentration of heavy metal ions in the atomicadsorption analysis of background contents in drinking water, in: W. Fluch (Ed.), Problemy Inzynerii Srodoviska. Material XXIII Miezdunar. Sympoz. im. Boleslawa Krzistofica AQUA-2003, Politechnica Warsawska, Plock, 2003, p. 16.

V.S. Soldatov et al. / Reactive & Functional Polymers 71 (2011) 49–61 [28] L.A. Orlovskaya, V.S. Soldatov, Proc. Natl. Acad. Sci. Belarus, Chem. Ser. 4 (2004) 100–104. [29] T. Yoshioko, M. Shimamura, Bull. Chem. Soc. Jpn. 58 (1985) 2618–2625. [30] L.A. Orlovskaya, Proc. Natl. Acad. Sci. Belarus, Chem. Ser. 2 (2003) 42–44. [31] V.S. Soldatov, Solvent Extract. Ion Exchange 26 (2008) 457–513. [32] O.M. Vatutsina, V.S. Soldatov, React. Funct. Polym. 67 (2007) 184–201. [33] V.S. Soldatov, Z.I. Sosinovich, T.A. Korshunova, T.V. Mironova, React. Funct. Polym. 58 (2004) 3–12. [34] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, et al., J. Comput. Chem. 14 (1993) 1347–1363. [35] S. Huzinaga, J. Andzelm, M. Klobukowski, Gaussian Basis Sets for Molecular Calculations, Elsevier, Amsterdam, 1984. [36] V.S. Soldatov, V.M. Zelenkovskii, T.V. Bezyazychnaya. Computer modeling of hydration of the functional groups in ion exchangers, in: M. Cox (Ed.), Ion

[37] [38] [39] [40] [41] [42] [43]

61

Exchange Technology for Today and Tomorrow (Proc. IEX’04), Cambridge, University of Hertfordhire, 2004. pp. 365–372. V.S. Soldatov, Z.I. Sosinovich, T.V. Mironova, React. Funct. Polym. 58 (2004) 13– 26. A.W. Davidson, Ann. NY Acad. Sci. 57 (1953) 105–115. J. Krasner, J. Marinsky, J. Phys. Chem. 12 (1963) 2559–2561. A. Yuchi, K. Mukaie, Y. Sotomura, et al., Anal. Sci 18 (2002) 575–577. W.J. Weber, S. Morris, J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 89 (1963) 31– 59. V.D. Kopylova, A.N. Astanina, in: Khimia, (Ed.), Ion Exchange Complexes in Catalysis, Moscow, 1980, 190 p (in Russian). R.M. Diamond, D.C. Whitney, Resin selectivity in dilute and concentrated aqueous solutions, in: J.A. Marinsky (Ed.), Ion Exchange, vol. 1, Marcel Dekker Inc., New York, 1966, pp. 277–351.