Simulation of copper (II) cations sorption on goethite from aqueous ...

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2017, Vol. 62, No. 2, pp. 150–159. © Pleiades Publishing, Ltd., 2017. Original Russian Text © T.N. Kropacheva, A.S. Antonova, V.I. Kornev, 2017, published in Zhurnal Neorganicheskoi Khimii, 2017, Vol. 62, No. 2, pp. 155–164.

SYNTHESIS AND PROPERTIES OF INORGANIC COMPOUNDS

Simulation of Copper(II) Cations Sorption on Goethite from Aqueous Solutions of Complexons T. N. Kropacheva*, A. S. Antonova, and V. I. Kornev Udmurt State University, Izhevsk, 426034 Russia *e-mail: [email protected] Received December 1, 2015

Abstract―Effect of complexons of polyaminopolycarboxylic acid series (IDA, NTA, EDTA, and DTPA) and polyphosphonic acid series (HEDP, NTP, and EDTP) on Cu(II) cations sorption on goethite (α-FeOOH) from aqueous solutions has been studied. Obtained results have been considered in the context of complexation reactions in bulk solution and on sorbent surface. It has been found that all complexons (except for EDTA), depending on nature, produce on goethite surface (≡FeOH) triple complexes of type A (surface– metal–complexon) of composition ≡FeOCuLH1i + i − n and ≡FeOCuL(OH)1j− j − n or type B (surface–complexon–metal) of composition ≡FeLHiCu3+i-n and ≡FeLCu(OH)3j − j − n. pH-Ranges for complex existence and stability constants for the surface complexes have been determined. Factors affecting the character of complexon effect (immobilization/remobilization) on the sorbed metal have been analyzed. DOI: 10.1134/S0036023617020103

Investigation of the character of complexon effect on the sorption of metal cations onto different sorbents allows one to establish relationship between coordination equilibria in solutions and surface processes. Accumulated large body of data on the coordination chemistry of complexons and metal complexonates in aqueous solutions [1] provides a good basis for transfer to more complicated heterogeneous system containing a metal cation, a complexon, and sorbent. The features of surface phenomena in metal cation– complexon–sorbent system are poorly studied in spite of the fact that they are of great importance for many practical tasks such as application of complexons and metal complexonates as corrosion and salt deposition inhibitors, employment of complexons for extractive purification of natural and technogenic sediments, sorption and preconcentration of metal ions, development of modified electrodes and catalysts based on immobilized metal complexonates, etc. There are literature data on the study of heterogeneous equilibria involving different metals (Cu(II), Ni(II), Pb(II), Zn(II), Cd(II), Pb(II), Co(II), Al(III), Fe(III), Ca(II), and others), complexons (EDTA, NTA, NTP, HEDP, etc.), and carrier phases (goethite, α-FeOOH, boehmite γ-AlOOH, γ-Al2O3, δ-Al2O3, gibbsite α-Al(OH)3, amorphous Fe(III) and Al(III) hydroxides, clay, and others) [2–9]. Formation of surface complexes of metal cation and complexon was shown to proceed in many systems along with decrease of metal sorption under the action of complexon. However, authors often describe sorption processes only

qualitatively or study only one particular system, which provides no possibility to reveal general rules. Therefore, the aim of this work is to study systematically the sorption of copper(II) cations in the presence of different complexons and to describe quantitatively the equilibria in Cu(II)–complexon–sorbent triple systems in the context of surface complexation theory (Surface Complexation Modeling) [10, 11]. Our tasks were to establish the composition and stability of surface complexes and to reveal the role of all factors affecting the interfacial distribution of metal cation, in particular, its desorption or immobilization. The structure and stability of Cu(II) complexonates are close to those for many other metals (Pb(II), Ni(II), Co(II), Zn(II), and Cd(II)), which imparts general character to our study. We used the most thermodynamically stable iron(III) oxyhydroxide, goethite α-FeOOH, a well-known natural and synthetic sorbent whose surface structure was characterized in detail in the literature [11, 12]. To establish relationship between stability and structure of Cu(II) complexonates in solutions and the character of complexon effect on Cu(II) sorption, we studied several most common complexons from polyaminopolycarboxylic acid series (IDA, NTA, EDTA, and DTPA) and polyphosphonic acid series (HEDP, NTP, and EDTP) (Table 1).

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EXPERIMENTAL Goethite (α-FеООН) was prepared by method [11] by mixing 250 mL of 0.1 mol/L Fe(NO3)3 solution and 100 mL of 2 mol/L KOH solution. The resultant precipitate was aged in mother liquor at 70°C for 60 h, washed with water to pH ≈ 7, separated by filtration, dried, and stored in air. The X-ray powder diffractogram of the obtained sample (DRON-6 diffractometer) coincides with data reported for goethite in different crystallographic databases. The specific surface area of goethite was determined from low-temperature nitrogen sorption isotherms by four-point BET method (Sorbi-M® instrument). Prepared goethite samples had specific surface area in the range 51–82 m2/g. IR spectrum of goethite (Excalibur HE 3100 Varian BV IR Fourier-transform spectrometer) shows absorption bands at 3450 and 1633 cm–1, related to the stretching and deformational vibrations of OH groups of water sorbed on the surface. Typical for goethite absorption bands at 893, 794, and 641 cm–1 are responsible for the deformational vibrations of Fe– OH and Fe–O–Fe bonds [11]. Solutions of complexons with concentration of 0.1 mol/L were prepared by dissolution of weighed sample of chemicals (reagent grade) in distilled water followed by determination of strict concentration by pH-metric titration. A solution of Cu(II) salt was obtained by dissolution of a weighed sample of Cu(NO3)2 · 3H2O (reagent grade) in distilled water, the strict concentration of the resultant solution was determined by complexometric titration. The sorption of Cu(II) cations was studied in static mode at continuous shaking for 1 h in the presence of background electrolyte (0.1 mol/L KNO3). Necessary medium acidity was made and maintained by HNO3 and KOH solutions. The content of reagents in sorption experiments was constant: 10–4 mol/L (Cu(II) and complexons) and 1 g/L (goethite). After sorption completed, goethite was separated by filtration through paper filter (blue ribbon) and pH of equilibrium solution was measured on an I-160MI ionometer. Residual Cu(II) concentration in solutions after sorption was determined by spectrophotometric method (SF-2000 spectrophotometer) using reaction with sodium diethyldithiocarbamate to form a colored compound. It was found preliminary that the presence of the studied complexons in solution (up to tenfold excess) did not hamper Cu(II) determination. Sorption degree (in %) was calculated by the formula: С − С eq Г(%) = 0 × 100, where C0 and Ceq are initial С0 and equilibrium metal concentrations in solution, respectively. The statistical treatment of results of concurrent sorption experiments showed that Cu(II) determination error was not larger 5%. All experiments were carried out at 25°С. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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Sorption equilibria were simulated with the use of free (VisualMinteq [13] and Hyss [14]) and proprietary (FITEQL [15]) programs, which allow modeling equilibria in solutions and surface equilibria using non-electrostatic and electrostatic models of sorbents. RESULTS AND DISCUSSION For quantitative description of sorption processes in Cu(II)–goethite and Cu(II)–complexon–goethite systems, we used surface complexation theory (Surface Complexation Modeling), which is widely used to describe cation and anion sorption on different (hydr)oxides, iron (hydr)oxides including [10–12]. On the formation of inner-sphere surface complexes (specific sorption), the energy of chemical reaction (formation of covalent bond between sorbate and surface functional groups) prevail over electrostatic component caused by surface charge. In this case, one can use the simplest non-electrostatic model of the theory where sorption processes are described similarly to homogeneous equilibria in solutions without regard for electrostatic factor; this approach was used in the present work. Surface OH groups are the sorption centers of goethite, the acid–base properties of these groups were studied by potentiometric titration of goethite suspension with standard solutions of acid/alkali by the common procedure [11]. For surface reactions:

≡FeOH ↔ ≡FeO– + H+ (logK = –8.2 ± 0.6), ≡FeOH + H+ ↔ ≡ FeOH 2+ (logK = 5.9 ± 0.2). The obtained constants are within typical values for goethite [11, 12]. The total concentration of OH groups involved in sorption (TFeOH) necessary for calculations was assessed with allowance made for goethite specific surface area (51–82 m2/g) and known content of monocoordinated (terminal) OH groups in goethite (three groups per nm2). For these values, ТFeOH is within 0.26–0.41 mmol/g, which is comparable with values used in other works [2, 3, 11, 12]. The sorption of Cu(II) cations on goethite in the absence of complexons increases when medium acidity decreases (Figs. 1–7). Sorption curve may be well described by the formation of one surface complex according to scheme: ≡FeOH + Cu2+ ↔ ≡FeOCu+ + H+ (logK = –1.6 ± 0.2), as was established previously for the sorption of many other doubly charged metal cations on different (hydr)oxides [2–6, 11, 12]. The modeling of heterogeneous equilibria in Cu(II)–complexon–goethite system requires information on the constants of homogeneous equilibria: dissociation (protonation) constants for complexons and stability constants for Cu(II) complexonates.

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Table 1. Equilibrium constants for the reactions of complexons and Cu(II) cations in aqueous solutions(25°С, I = 0.1) [16] Complexon

Reaction

IDA (H2L) HOOC

N H

COOH

NTA (H 3L) HOOC N

EDTA (H 4L) COOH

HOOC N

COOH

HOOC DTPA (H 5L)

COOH HOOC

N

N

N

COOH

COOH COOH

HEDP (H4L) PO(OH)2 H3 C

C

(HO)2OP PO(OH)2

10.56 16.3

HL– + H+ = H2L

2.62

Сu2+ + 2L2– = CuL22− CuL + H+ = CuHL+ CuOHL– + H+ = CuL + H2O

L3– + H+ = HL2–

9.66

Сu2+ + L3– = CuL–

13.0

2.52

Сu2+ + 2L3– = CuL42− CuL– + H+ = CuHL

17.4

HL2– + H+ = H2L– H2L– + H+ = H3L

1.81

CuOHL2– + H+ = CuL– + H2O

L4– + H+ = HL3–

10.19

2.3 8.5

1.6

PO(OH)2

(HO)2OP N

PO(OH)2

18.78

6.31

CuL2– + H+ = CuHL–

3.1

H2L2– + H+ = H3L–

2.69

CuHL – + H+ = CuH2L

2.0

H3L– + H+ = H4L

2.0

CuOHL3– + H+ = CuL2– + H2O 11.4

10.5 L5– + H+ = HL4– 4– + 3– 8.60 HL + H = H2L H2L3– + Н+ = H3L2– 4.28 H3L2– + H+ = H4L– 2.70

CuL3– + H+ = CuHL2–

4.80

CuHL2– + H = CuH2L–

2.96

H4L– + H+ = H5L

CuL3– + Cu2+ = Cu2L–

6.79

2.0

Сu2+ + L5– = CuL3–

21.2

10.9 L4– + H+ = HL3– HL3– + H+ = H2L2– 6.88 H2L2– + H+ = H3L– 2.60

CuL2– + H+ = CuHL–

6.9

H3L– + H+ = H4L

CuHL – + H+ = CuH2L

4.0

L8– + H+ = HL7–

EDTP (H 8L)

Сu2+ + L4– = CuL2–

9.2

HL3– + H+ = H2L2–

H4L2– + H+ = H5L–

(HO)2OP

(HO)2OP

Сu2+ + L2– = CuL 9.34

1.4

12.7 L6– + H+ = HL5– 5– + 4– 7.22 HL + H = H2L H2L4– + H+ = H3L3– 5.85 H3L3– + H+ = H4L2– 4.62

NTP (H 6L)

N

logK

L2– + H+ = HL–

OH

PO(OH)2

N

Reaction

COOH

HOOC

N

logК

HL7– + H+ = H2L6– H2L6– + H+ = H3L5– H3L5– + H+ = H4L4– H4L4– + H+ = H5L3– H5L3– + H+ = H6L2– H6L2– + H+ = H7L– H7L– + H+ = H8L

1.5

Сu2+ + L4– = CuL2–

12.1

17.3

Сu2+ + L6– = CuL4– CuL4– + H+ = CuHL3– CuHL3– + H+ = CuH2L2– CuH2L2– + H+ = CuH3L– CuH3L– + H+ = CuH4L

6.36 4.55 3.48 1.4

13.0 9.83 7.89 Сu2+ + L8– = CuL6– 6.40 CuL6– + H+ = CuHL5– 5.15 CuHL5– + H+ = CuH2L4– 2.95 CuH2L4– + H+ = CuH3L3– 1.3 CuH3L3– + H+ = CuH4L2– –

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(а)

80

2

60 40

80

40 20

0

0 2

3

4

5

6

7

8

1

9 10 11 pH

(b)

100

2

60

20

1

1

Exp. Exp. Calcd.

100 Sorption, %

Sorption, %

(а)

1

Exp. Exp. Calcd.

100

153

2

3

4

5

6

7

8

9 10 11 pH

(b)

100

5 6

80

2

60

Molar fraction, %

Molar fraction, %

80

4

3

40 20

7

8

1 0

7 4

60 3

40

2 5

20

6

1

0 0

1

2

3 4

5

6

7

8

9 10 11 pH

0

Fig. 1. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of IDA at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–IDA–goethite system: (1) ≡FeOCu+; (2) ≡ FeOCuL –; (3) ≡ FeOCuLН; (4) ≡FeOCuL(OH)2–; (5) Cu2+; (6) CuL; (7) CuНL+;

3

4

5

6

7

8

9

10 11 pH

Fig. 2. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of NTA at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–NTA–goethite system: (1) ≡FeOCuLH2; (2) ≡FeOCuLH–; (3) ≡FeOCuL2–; (4) ≡FeOCuL(OH)3–; (5) Cu2+; (6) CuHL; (7) CuL– (b).

(8) СuL22− (b).

The values of these constants (Table 1) were taken from the most reliable sources [16] and used in modeling as fixed values. The comparison of sorption curves in the absence and in the presence of equimolar amount of complexon (Figs. 1–7) shows that decrease of metal sorption is observed for all complexons in wide pH range due to competitive complexation reaction in solution. Computations display that the sorption of Cu(II) cations should be decreased to zero level within all pH range under the action of all complexons (except for IDA). However, experimental data indicate that this behavior is observed only for EDTA (Fig. 3), whereas the drop of Cu(II) sorption in the presence of other complexons is not so large, while DTPA (Fig. 4) and all phosphonic complexons (Figs. 5–7) even increase Cu(II) sorption in acidic medium. It is obvious that the explanation of obtained data should take into account the possibility of Cu(II) complexonates RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

2

formed in solution to bind to the surface. In terms of surface complexation theory [10–12], the cooperative sorption of metal cations and ligands is considered as a reaction of formation of triple surface complexes by two mechanisms: (1) due to reaction of metal cation bound to ligand with surface (complexes of type A: surface–metal–ligand), (2) due to reaction of ligand bound to metal cation with surface (complexes of type B: surface–ligand–metal). The formation of the surface complexes of type A is possible only when ligand could not saturate metal coordination sphere, while the complexes of type B, on the contrary, appear when ligand has functional groups not involved in coordination to metal. Let us consider obtained results on the effect of different complexons on Cu(II) cation sorption from this point of view. In the modeling of sorption processes (along with homogeneous protolytic and coordination equilibria in solutions), we made allowance, depend-

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(а)

80 60 40 20

80 60 40

2

20

2

0

0 1

2

2

3

4

5

6

7

8

9 10 11

1

2

3

4

5

pH

(b) 100

6

7

8

9 10 11 pH

(b)

100 1

6

2 80 Molar fraction, %

80 Molar fraction, %

1

Exp. Exp. Calcd.

100 Sorption, %

100 Sorption, %

(а)

1

Exp. Exp. Calcd.

60 40 3

4 20

8 60

7

40

1

2 20

4

5

3

0

0 0

1

2

3 4

5

6

7

8

9 10 11 pH

Fig. 3. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of EDTA at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–EDTA–goethite system: (1) Cu2+; (2) CuL2–; (3) CuНL–; (4) СuН2L (b).

ing on considered system, for the following surface reactions (Table 2): (1) the formation of complexes of type A: ≡FeOH + Cu2+ + Ln– + iH+ ↔ ≡FeOCuLH1i +i − n + H+, or in terms of complexonate sorption: ≡FeOH + CuLH i2 +i − n ↔ ≡FeOCuLH1i +i − n + H+ (i = 0, 1… n – 1); (2) the formation of complexes of type B: ≡FeOH + Cu2+ + Ln– + (1 + i)H+ ↔ ≡FeLHiCu3 + i – n + H2O, or in terms of complexonate sorption: ≡FeOH + CuLH i2 +i − n + H+ ↔ ≡FeLHiCu3 + i – n + H2O (i = 0, 1… n – 1).

1

2

3

4

5

6

7

8

9 10 11 pH

Fig. 4. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of DTPA at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–DTPA–goethite system: (1) ≡FeLCu2–; (2) ≡FeLHCu–; (3) ≡FeLН2Cu; (4) ≡FeLCu(OH)3–; (5) ≡ FeLCu(OH)42 −, (6) CuL3–; (7) CuНL2–; (8) CuH2L – (b).

Moreover, the possibility of hydroxo complexes formation in alkaline medium was taken into consideration:

≡FeOH + Cu2+ + Ln– + jH2O ↔ ≡FeOCuL(OH)1j− n− j + (1 + j)H+ (j = 1, 2); ≡FeOH + Cu2+ + Ln– + (j – 1)H2O ↔ ≡FeLCu(OH)3j − n− j + (j – 1)H+ (j = 1, 2). A minimal set of surface complexes satisfactorily describing experimental sorption curve was considered for each system. The mechanism considered upon modeling implies the formation of only mononuclear surface complexes where bridging complexon ligand binds to one Fe(III) ion on goethite surface. Similar approach was used by the authors of many works [2–6] and was confirmed by the spectral studies of sorption of carboxylic and phosphonic acids, com-

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40

2

80

2 60 40

20

20

0

0 0

1

2

3

4

5 6

7

8

9 10 11 pH

0

(b)

100 6

8

3 9 10

1

20

2

4

5

6

7

8

9 10 11 pH

8 9 2

60

13

1 7

40

12

20

5

2

3

(b)

80

60 40

1

100

7

80 Molar fraction, %

Sorption, %

60

1

Exp. Exp. Calcd.

100

Molar fraction, %

Sorption, %

80

(а)

1

Exp. Exp. Calcd.

100

155

4 11 3

10

5

6

4 0

0 0

1

2

3 4

5

6

7

8

9 10 11 pH

0

Fig. 5. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of NTP at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–NTP–goethite system: (1) ≡FeLCu3–; (2) ≡FeLHCu2–; (3) ≡FeLН2Cu–;

2

3 4

5

6

7

8

9 10 11 pH

Fig. 6. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of EDTP at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–EDTP–goethite system: (1) ≡FeLCu5–; (2) ≡FeLHCu4–; (3) ≡FeLН2Cu3–; (4) ≡FeLН3Cu2–; (5) ≡FeLН4Cu–; (6) ≡FeLCu(OH)6–;

(4) ≡FeLCu(OH)4–; (5) ≡ FeLCu(OH)52−, (6) Cu2+; (7) CuL4–; (8) CuНL3–; (9) Cu H2L2–; (10) CuH3L – (b).

(7) ≡ FeLCu(OH)72 −, (8) Cu2+; (9) CuL6–; (10) CuНL5–; (11) CuH2L4–; (12) CuH3L3–, (13) CuH4L2– (b).

plexons including, on different (hydr)oxides [11, 12, 17, 18]. Cu(II)–IDA–goethite and Cu(II)–NTA–goethite systems. These monoamine (poly)carboxylic complexons cause decrease of Cu(II) sorption on goethite in neutral and alkaline medium owing to formation of neutral Cu(II) complexonates in solution (Figs. 1 and 2). The stability of NTA complex is higher than that of IDA complex (Table 1), therefore the desorbing effect of NTA is more pronounced. Since these complexes have insufficient denticity to saturate Cu(II) coordination sphere (IDA is tridentate ligand, NTA is tetradentate one), both normal and protonated Cu(II) complexonates are sorbed on goethite surface by the type A (Table 2). Normal Cu(II)–IDA complexes have higher sorption ability and their share on the surface is much larger than that of similar Cu(II)–NTA surface complexes. As a whole, the sorption of normal RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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Cu(II) complexonates on goethite proceeds much worse than that of free (hydrated) Cu(II) cations. Taking into account the known structure of Cu(II) complexonates with IDA and NTA in solutions [1], one can suppose that, upon binding of normal complexonates to the surface, coordinatively unsaturated Cu(II) cation forms one additional bond with goethite oxygen atom and remains involed into two (IDA) or three (NTA) glycinate metallocycles (Fig. 8). In alkaline medium, hydroxo complexes also form on the surface along with normal complexonates, the hydrolysis of complexonates, like in solutions (Table 1), proceeds better in the case of IDA. Cu(II)–EDTA–goethite system. In the presence of this hexadentate diamino(poly)carboxylic complexon, we observed a decrease of Cu(II) sorption to zero level within all pH range (Fig. 3). This is due to formation

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(а) 100 Sorption, %

1

Exp. Exp. Calcd.

80

C FeO

Cu

N

60 O

40

C

C

2

C

O

20 O

0

А 0

1

2

3

4

5

6

7

8

9 10 11 pH

O O

4

80

O P

(b)

100

Molar fraction, %

C

O

5

P Fe

O C

60

O

O C N

Cu C

O P

2 40

O

6 B

20

3

1

7

Fig. 8. Supposed structure of normal surface complexes: complex of type A: Cu2+–NTA–goethite (≡FeOCuL2–); complex of type B: Cu2+–NTP–goethite (≡FeLCu3–).

0 0

1

2

3

4

5

6

7

8

9 10 11 pH

Fig. 7. Cu(II) sorption on goethite in the absence (1) and in the presence (2) of HEDP at different medium acidity (a). Molar distribution of Cu(II) in Cu(II)–HEDP–goethite system: (1) ≡FeOCu+; (2) ≡ FeLCu–; (3) ≡ FeLНCu; (4) Cu2+; (5) CuL2–; (6) CuНL–; (7) CuH2L (b).

of stable normal complexonate CuL2–, where Cu(II) coordination sphere is completely saturated with the ligand, which prevents its sorption by type A. At the same time, EDTA has no free functional groups to produce bonds to surface, which prevents its sorption by type B as well. Thus, Cu(II)–EDTA complexonates provide a good example for how complexation in solution can fundamentally change the sorption properties of metal cation. Cu(II)–DTPA–goethite system. The desorbing effect of DTPA on Cu(II) cations is expressed much less than for EDTA (Fig. 4), in spite of the higher stability of DTPA complexes in solution (Table 1). Moreover, DTPA in acidic medium even increases Cu(II) sorption, which is not observed for other (poly)aminopolycarboxylates. The probable reason of such an anomalous effect for this octadentate complexon is the structure of complexonates formed in solution where DTPA completely saturates Cu(II) coordination

sphere thus excluding the formation of complexes of type A. One of carboxylic groups not involved into coordination to Сu(II) [1] can bind to the surface to produce normal and protonated complexes of type B (Table 2). Cu(II)–NTP–goethite system. This tetradentate complexon is a phosphonic analog of NTA (carboxylic groups–COOH in NTA are replaced in NTP molecule by phosphonic groups –PO(OH)2) (Table 1). However, NTP–Cu(II) complexes can be sorbed by type B due to extremely strong sorption interaction with the surface of different metal (hydr)oxides (due to formation of ≡M–OPO(OH) strong covalent bond) [17]) inherent in all phosphonic complexons. Calculations showed that the largest contribution to Cu(II) immobilization at pH 2–5 is provided by protonated surface complex ≡FeLH2Cu– (Fig. 5). In more alkaline medium, a normal comlex ≡FeLCu3– exists, which is converted into hydroxo complexes ≡FeLCu(OH)4– and ≡FeLCu(OH)52−. on account of coordination unsaturation of Cu(II). NTP molecule in normal complex is likely to bind to the surface via one of phosphonic groups, while remaining phosphonic groups coordinate to Cu(II) to form two nitrogen-containing five-membered rings (Fig. 8). Cu(II)–EDTP–goethite system. EDTP is a phosphonic analog of EDTA but it does not cause such a

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Table 2. Equilibrium constants for reactions (logK*) with participation of complexons and Cu(II) cations on goethite surface (25°С, I = 0.1) Complexes of type A Reaction

IDA

NTA

≡FeOH + Cu2+ + Ln– ↔ ≡FeOCuL1 – n + H+

6.8

8.4

≡FeOH + CuL2 – n ↔ ≡FeOCuL1 – n + H+

–3.8

–4.6

≡FeOH + Cu2++ Ln– ↔ ≡FeOCuLH2 – n

14.4

16.2

1.5

1.6

–

20.9

–3.7

–2.5

–14.3

–15.5

≡FeOH + CuLH3 – n ↔ ≡FeOCuLH2 – n + H+ n ≡FeOH + Cu2+ + Ln– + H+ ↔ ≡FeOCuLH 3– 2

≡FeOH + Cu2+ + Ln– + H2O ↔ ≡FeOCuL(OH)–n + 2H+ ≡FeOH + CuL2 – n + H2O ↔ ≡FeOCuL(OH)–n + 2H+ Complexes of type B Reaction

DTPA

≡FeOH + Cu2+ + Ln– + H+ ↔ ≡FeLCu3 – n + H2O ≡FeOH + CuL2 – n + H+ ↔ ≡FeLCu3 – n + H2O ≡FeOH + Cu2+ + Ln– + 2H+ ↔ ≡FeLHCu4 – n + H2O ≡FeOH + CuLH3 – n + H+ ↔ ≡FeLHCu4 – n + H2O ≡FeOH + Cu2+ + Ln– + 3H+ ↔ ≡FeLH2Cu5 – n + H2O

HEDP

EDTP

30.6

22.5

27.2

34.9

9.4

10.4

9.9

11.7

35.3

27.0

32.3

41.5

9.3

7.0

8.6

10.7

38.0

46.2

9.8

9.5

37.5 –

≡FeOH + CuLH 42 − n + H+ ↔ ≡FeLH2Cu5 – n + H2O

8.5

≡FeOH + Cu2+ + Ln– + 4H+ ↔ ≡FeLH3Cu6 – n + H2O

50.8 –

≡FeOH + CuLH 35− n + H+ ↔ ≡FeLCu6 – n + H2O

–

– 9.4

≡FeOH + Cu2+ + Ln– + 5H+ ↔ ≡FeLH4Cu7 – n + H2O

53.7 –

≡FeOH + CuLH 64− n + H+ ↔ ≡FeLH4Cu7 – n + H2O

–

– 8.6

≡FeOH + Cu2+ + Ln– ↔ ≡FeLCu(OH)2 – n ≡FeOH + CuL2 – n ↔ ≡FeLCu(OH)2 – n

24.4 3.2

≡FeOH + Cu2+ + Ln– + H2O ↔ ≡FeLCu(OH)12− n + H+

15.9

≡FeOH + CuL2 – n + H2O ↔ ≡FeLCu(OH)12− n + H+

–5.3

–

19.9 2.6

26.6 3.4

11.8

18.3

–5.5

–4.9

–

* The error for the given values of logK varies from 0.1 to 0.3 depending on the system. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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KROPACHEVA et al.

considerable desorption of Cu(II) cations (Fig. 6) in spite of higher stability of Cu(II)–EDTP complexonates than similar in composition Cu(II)–EDTA complexonates (Table 1). The high denticity of EDTP in combination with the presence of four phosphonic groups, which provide good binding to the surface, causes the highest (among the complexones under consideration) probability to form surface complexes of type B that include EDTP. The calculations show that Cu(II) immobilization in acidic medium is associated with a set of complexes with different protonation degree of ≡FeLHiCui – 5 ligand (i = 0–4). Cu(II)–HEDP–goethite system. The results of calculations show that the strongly pronounced desorbing effect of HEDP on Cu(II) in alkaline medium is caused by the existence of normal CuL2– complexonate in solution (Fig. 7). The immobilization of Сu(II) cations under the action of HEDP at pH 4–6 takes place due to formation of triple surface complex of type B of composition ≡FeLCu–, where HEDP is likely to play a role of a bridge and binds to Fe(III) cations of goethite via one of phosphonic groups and to Cu(II) cation via another group. This fact is in agreement with the known ability of HEDP to produce polynuclear complexonates (homo- and heteronuclear) in solutions and crystal state [1]. As a whole, the results of the work show that, along with formation of metal complexonates in solution in heterogeneous system metal cation–complexon–surface, the fixation of the complexonates on the surface occurs. The obtained results agree well with the data of other authors who showed the possibility to form surface complexes of type B in the systems M(II)– ETDA–goethite (where M(II) is Cu(II), Zn(II), Pb(II), Co(II), Ni(II) [2], Ni(II)–ETDA–sorbent (ferrihydrite, lepidocrocite, γ-Al2O3) [3], Сu(II)– NTP– boehmite [6], Cu(II)–HEDP–γ-Al2O3 [5]). Stronger sorption of Cu(II) cations within complexes in acidic media as compared with the free cations in the presence of certain complexons results from the formation of complexes of type B (Figs. 4–7). The relationship between complexonate content in homogeneous and heterogeneous state is dependent first of all on the stability and structure of metal complexonate in solution and on medium acidity. Among all studied complexons, only EDTA produces no Cu(II) complexonates capable of binding to goethite surface, which is explained by their structure discussed above. The character of binding of other Cu(II) complexonates to goethite is dependent on complexon nature. Aminopolycarboxylic complexons with denticity lower than 6 (IDA, NTA) produce complexes of type A due to incomplete saturation of metal coordination sphere with these ligands. Polyaminopolycarboxylates with denticity higher than 6 (DTPA) produce no complexes of type A but they form complexes of type B due to the presence of free carboxylic groups. All phosphonic complexons (HEDP, NTP, EDTP) also form

only surface complexes of type B due to the high affinity of phosphonic group to goethite surface. The constants of surface complex formation in the series of phosphonic complexons according to scheme ≡FeOH + Cu2++ Ln– + H+ ↔ ≡FeLCu3 – n + H2O increase considerably with the number of phosphonic groups in complexon molecule (HEDP < NTP < EDTP), which correlates with the growth of stability of CuL2 – n complexonates in solution. The comparison of intrinsic sorption ability of Cu(II) complexonates can be made more correctly using reaction ≡FeOH + CuL2 – n + H+ ↔ ≡FeLCu3 – n + H2O, whose equilibrium constant is weakly dependent on phosphonate nature (Table 2). This indicates that, in spite of considerable difference in structure, all phosphonic acids binds to the surface by similar mechanism, namely, with participation of only one phosphonic group. Normal Cu(II)–DTPA complex binds to the surface worse than normal Cu(II) phosphonates, which is explained by lower strength of bond between Fe(III) of goethite and the carboxylic group of the complexon as compared with phosphonic group. The surface complexes of type A form better at high solution pH (similarly to the sorption of free metal cations) while complexes of type B are more stable at lower pH (similarly to ligand sorption). The obtained results display that natural and technogenic systems containing different Fe(III) oxides, oxyhydroxides, and hydroxides can provide (re)mobilization of sorbed Cu(II) cations (and other heavy metal cations showing similar sorption and complexation properties) under the action of complexons. The desorbing effect in neutral and weakly alkaline medium is typical for all complexons to some extent but especially for EDTA and DTPA, which is a source of environmental hazard related to accidental release of these complexons to the environment [19]. At the same time, the efficient desorption of Cu(II) cations under the action of EDTA can provide a good basis for the extractive method of purification of different soils, sludges, etc. contaminated by heavy metals [8, 20, 21]. Thus, (poly)aminopolycarboxylic and phosphonic complexons are efficient regulators of heterogeneous processes, which can be used to increase or decrease the surface-fixed state of metal cations. REFERENCES 1. N. M. Dyatlova, V. Ya. Temkina, and K. I. Popov, The Complexones and Metal Complexonates (Khimiya, Moscow, 1988) [in Russian]. 2. B. Nowack and L. Sigg, J. Colloid Interface Sci. 177, 106 (1996). 3. B. Nowack, J. Lützenkirchen, P. Behra, and L. Sigg, Environ. Sci. Technol. 30, 2397 (1996). 4. B. Nowack and A. T. Stone, Environ. Sci. Technol. 33, 3627 (1999). 5. L. Hein, M. C. Zenobi, and E. Rueda, J. Colloid Interface Sci. 314, 317 (2007).

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SIMULATION OF COPPER(II) CATIONS SORPTION 6. M. C. Zenobi and E. H. Rueda, Quim. Nova 35, 505 (2012). 7. A. S. Antonova, T. N. Kropacheva, M. V. Didik, and V. I. Kornev, Sorbtsionnye i Khromatograficheskie Protsessy 14 (2), 65 (2014). 8. T. N. Kropacheva, A. S. Antonova, Yu. V. Rabinovich, and V. I. Kornev, Russ. J. Appl. Chem. 87, 1422 (2014). 9. K. Güçlü and R. Apak, J. Colloid Interface Sci. 228, 238 (2000). 10. Surface Complexation Modelling, Ed. by J. Lutzenkirchen, (Academic Press, 2006). 11. R. M. Cornell and U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses (Wiley-VCH, 2003). 12. D. A. Dzombak and F. M. M. Morel, Surface Complexation Modelling: Hydrous Ferric Oxide (John Wiley and Sons, 1990). 13. www2.lwr.kth.se/English/OurSoftware/vminteq/(Visual MINTEQ, ver.3.0).

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14. www.hyperquad.co.uk/hyss.htm (HySS2009/Hyperquad Simulation and Speciation). 15. A. Herbelin and J. Westall, FITEQL: a computer program for determination of chemical equilibrium constants from experimental data, version 4.0. 16. www.nist.gov/srd/nist46.cfm (NIST Standard Reference Database 46. Critically Selected Stability Constants of Metal Complexes: Version 8.0). 17. C. Queffélec, M. Petit, P. Janvier, et al., Chem. Rev. 112, 3777 (2012). 18. M. C. Zenobi, C. V. Luengo, M. J. Avena, et al., Spectrochim. Acta A 75, 1283 (2010). 19. T. P. Knepper, Trends Anal. Chem. 22, 708 (2003). 20. D. Leštan, C.-L. Luo, and X.-D. Li, Environ. Pollut. 153, 3 (2008). 21. G. N. Koptsik, Euras. Soil Sci. 47, 707 (2014).

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Translated by I. Kudryavtsev

No. 2

2017