Solvent extraction of chromium and copper using Schiff base derived from terephthaldialdehyde and 5-amino-2-methoxy-phenol

2.1 Reagents and apparatus

Chloroform, nitrobenzene, CuCl₂·2H₂O, CrCl₃·6H₂O, KCl, NaH₂PO₄ and NaOH, were the analytical grade reagents and were purchased from Merck. 5-Amino-2-methoxy-phenol and terephthaldialdehyde were obtained from Fluka. Distilled water was used in extraction experimentals. The solvents were saturated with each other before use in order to prevent volume changes of the phases during extraction. The C, H and N were analyzed on a Carlo-Erba 1106 elemental analyzer. IR spectra were recorded on a Midac 300 instrument in KBr pellets. Mass spectra were recorded using a KRATOS MS50TC spectrometer. AA 929 Unicam Spectrometer was used for FAAS measurements with an air–acetylene flame. A pH meter (Metrohm 691 pH Meter) was also used. All extraction were performed by using a mechanical flask agitator in 50 cm³ stoppered glass flasks.

2.2 Ligand preparation

The preparation of ligand containing nitrogen and oxygen donor atoms is shown in Scheme 1. The structure of the new compound was characterized by a combination of IR and MS spectral data. The compound was synthesized according to the reported procedure (Reddy et al., 2009).

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Scheme 1

Synthesis of Schiff base.

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In a typical procedure, to a suspension of terephthaldialdehyde (1.34 g, 10 mmol) in 15 ml of aqueous methanol solution was added to a 15 ml solution of 5-amino-2-methoxy-phenol (2.78 g, 20 mmol) with stirring at 60 °C. The reaction mixture was continuously stirred for 30 min to separate the solid from reaction mixture and cooled. The solid was filtered and washed with diethyl ether. The corresponding product was recrystallized from methanol and found to be TLC-pure in chloroform and methanol mixture and dried under vacuum over P₄O₁₀. Yield% 97, m.p 236–238 °C. Elemental analysis found % (found atomic mass 376 amu), %, C: 70.1; H: 5.3; N: 7.4 for C₂₂H₂₀N₂O₄.

The electron impact spectrum of the ligand confirms the probable formula by showing a peak at 376 amu, corresponding to the macrocyclic moiety [(C₂₂H₂₀N₂O₄), calculated atomic mass 376]. The series of peaks in the range i.e. 15, 42, 59, 76, 82, 104, 110, 172, 186, 262, 290, 314 amu etc., may be assigned to various fragments.

The advantages of synthetic procedure are: (1) the process is a simple one – step reaction involving inexpensive starting materials: (2) the process is shorter: (3) using water methanol (1:1) as solvent allows the formation of imine without the need for catalysis and (4) yield is high and the reaction is fast, and the product can be isolated by filtration.

2.3 Extraction procedure

Aqueous solutions containing 1.5 × 10⁻³ M metal chloride in appropriate buffer were equilibrated with equal volumes of the chloroform and nitrobenzene solutions of the ligand 4 × 10⁻⁴ M by shaking in a mechanical shaker at 25 °C. Optimum equilibration time was determined for this system. In most cases distribution equilibrium was attained in less than 120 min and a shaking time of 120 min. The ionic strength of the aqueous phase was 0.1 M KCl in all experiments except those in which the effect of ionic strength was studied. After agitation, the solutions were allowed to stand for 120 min. The copper(II) and chrome(III) concentrations of the aqueous phase were determined by FAAS, and that of the organic phase from the difference by considering the mass balance. The pH of aqueous phase was recorded as equilibrium pH.

3.1 Extraction of metal ions with Schiff bases

3.1.1

3.1.1 Effect of pH and solvents on the extraction of Cu(II) and Cr(III)

Various organic solvents as inert diluents were tested at a fixed pH containing equal amount of metal chloride and ligand solutions. The phase volume ratio was always maintained at 1:1 to avoid emulsion formation. The diluents, nitrobenzene and chloroform are same effective. The exact cause of this type of behavior is not known. The dielectric constant of medium has some contribution in the extraction process. However the only factor determining the extraction efficiency in the extraction process can be taken into account and a better term correlating the relative extraction order is solubility parameter (Zolezzi et al., 1999; Percy and Thornton, 1973; Downing and Urbach, 1969; Sekine and Hasegawa, 1976).

The results indicate that H₂L in organic phase extracts efficiently Cu²⁺, Cr³⁺ aqueous phase containing 0.1 mol L⁻¹ KCl in the pH range of approximately 3–8.5 at 25 °C.

In this research liquid–liquid extraction experiments were performed to examine the efficiency and selectivity of Schiff base in transferring metal ions ( ${\mathrm{UO}}_2^{2+}$ ) and d-metal ions (Hg²⁺, Cu²⁺ and Cr³⁺) from aqueous phase into chloroform. The results show that metals ions of ${\mathrm{UO}}_2^{2+}$ and Hg²⁺ are not extracted by Schiff base (E < 1%).

The percentage extraction (%E) of some metals into chloroform and nitrobenzene with Schiff base were plotted as a function of aqueous phase pH equilibrated with the organic phase in Fig. 1. The results are also expressed as distribution ratio (Table 1). The distribution ratio of cation may be represented by Eq. (1).

(1)

$$D=\frac{{[\mathrm{MLA}]}_{\mathrm{org}}}{{[M^{+}]}_{\mathrm{aq}}}$$

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Figure 1

Effect of pH and Solvents on the extraction of [a] Cr(III), [b] Cu(II). [ ] CHCl₃, [ ] C₆H₅NO₂.

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Table 1 Distribution ratio of cation between the organic and aqueous phases.

pH	Distribution ratios of cations (D)a
	Cu2+		Cr3+
	Chloroform	Nitrobenzene	Chloroform	Nitrobenzene
3.27	0.016	0.067	0.53	0.74
4.12	0.22	0.096	0.65	0.78
5.08	0.23	0.15	0.63	0.64
6.27	65.66	41.85	1.16	1.00
7.05	34.29	28.5	10.42	3.79
7.57	19.86	20.68	8.4	5.6
8.15	19.45	15.21	4.7	3.8
8.4	21.22	10.18	1.4	5.6

a Averages calculated for data obtained from three independent extraction experiments.

The degree of separation was determined in terms of ‘separation factor’ S_f defined as the ratio of D₁ for the desired metal ion M₁ to D₂ for the contaminant metal ion M₂.

(2)

$$S_f=\frac{D_1}{D_2}$$

The selected relative cation selectivity of Cu²⁺/Cr³⁺ was calculated from the distribution ratio of metal ion between the organic and aqueous phase as shown in Table 2.

Table 2 The selected relative cation selectivities.

pH	Selectivity (Sf)
	Cu+2/Cr+3a
	Chloroform	Nitrobenzene
6.2	58	41.5
7.05	3.5	7.51
7.57	3	3.68
8.15	4.1	3.98
8.4	14	1.8

a Relative cation selectivity determined by the distribution ratio of metal ion between the organic and aqueous phases.

The extractability and selectivity of cations were evaluated as a function of pH. The highest extractability and selectivity for Cu²⁺ among the other used cations such as Cr³⁺ was achieved at pH 6.2. The selectivity of H₂L to Cu²⁺ over the Cr³⁺ is as follows Cu²⁺/Cr³⁺ = 58 for CHCl₃ and 41 for C₆H₅NO₂ (pH 6.2).

The extraction process may be represented by the equation:

(3)

$${\mathrm{M}}_{(\mathrm{w})}^{2+}+{\mathrm{H}}_2{\mathrm{L}}_{(0)}\leftrightarrow {\mathrm{ML}}_{(0)}+2{\mathrm{H}}_{(\text{w})}^{+}$$ Where H₂L represents the extractant reagent and subscript (w) and (0) denotes the aqueous and organic phases, respectively.

The extraction mechanism corresponds to a cation exchange, in which a complex of stoichiometric formula (CuL_n, CrL_n) is formed in the organic phase liberating at the same time n mol H⁺ ions in aqueous phase. In this case, extraction constant (K_ext) can be expressed as follows;

(4)

$$K_{\mathrm{ext}}=\frac{{[\mathrm{CuL}]}_o{[{\mathrm{H}}^{+}]}_{\text{w}}^{\text{n}}}{{[{\mathrm{Cu}}^{2+}]}_{\text{w}}{[{\mathrm{H}}_n\mathrm{L}]}_o}$$

When CuL is the only extractable species and the metal is present in the aqueous phase predominantly as the cation Cu²⁺, the metal distribution ratio (D) and the extraction constant are related by

(5)

$$\log D=\log K_{\mathrm{ext}}+n\mathrm{pH}+\log {[{\text{H}}_2\text{L}]}_0$$ According to Eq. (5) a plot of log D against pH at constant 4 × 10⁻⁴ M of [H_nL] which will give straight line of slope is the number of hydrogen ions and intercept log [H_nL] + log K_ext (Fig. 2).

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Figure 2

The plot of log D vs. pH at constant [H_nL] [a] Cu(II), [b] Cr(III). [ ] CHCl₃, [ ] C₆H₅NO₂.

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The effect of pH on the extraction of Cu²⁺ and Cr³⁺ ions from KCl media of ionic strength (I = 0.1 M) has been studied, the logarithm of the D values obtained were plotted against the corresponding pH values. A straight line with a slope of about 2 and 0.68 was obtained at I = 0.1 of Cu²⁺ and Cr³⁺, respectively, as shown Fig. 2. The values represent the number of hydrogen ions released during the formation of metal–ligand complex.

If the concentration of the extractants is constant and the hydrolysis in the aqueous phase as well as the polymerization in the organic phase occur to a negligible extent only, then the plots will be straight lines and their slopes will give the number of the ligands of the adducts.

Fig. 3 shows the evolution of log [D] when increasing the concentration of ligand at constant metal chloride concentration with two different organic solvents. As seen from the plots, there is a linear relationship between log [D] and log [L]_org, and the slope should be equal to the number of ligand molecules per cation in the extracted species. The slopes of lines are equal to 0.5 and 0.22 for dichloromethane and chloroform, respectively. Therefore, ligand forms a 2:1 (L:M) complex with Cu²⁺ and 3:1 (L:M) complex with Cr³⁺ for both solvents.

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Figure 3

The plot of log D vs. log [H₂L] at constant [H₂L] [a] Cu²⁺, [b] Cr³⁺. [ ] CHCl₃, [ ] C₆H₅NO₂.

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3.2 Effect of ionic strength on E%

The effect of ionic strength was studied at constant pH, a plot of the E% values versus the corresponding ionic strength values. The data (Fig. 3) indicate that the E% value increases with the increasing in KCl. This is explained by the increase of the thermodynamic activity of the metal salt extracted and decrease in the activity of water as the ionic strength increase, this is shown in Fig. 4(a, b), which means that Cl⁻ anion is functioning as salting out agent.

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Figure 4

The plot of E% vs. ionic strength and A/O at constant [H_nL] [a, c] Cu(II), [b, d] Cr(III). [ ] CHCl₃, [ ] C₆H₅NO₂.

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The effect of salt variation upon MCl_2,3 extraction by Schiff base was studied. The distribution ratio increases with the increase in salt concentration of KCl in ternary extraction system and this explained by the complexing ability of the anion.

3.3 Effect of aqueous to organic phase

Phase ratio (A/O) is one of the factors that affect the extraction efficiency. The extraction efficiency, E% can be represented by

where D is the distribution ratio, A and O are the volumes of the aqueous and organic phases, respectively. Equation indicates that the extraction efficiency decrease with increasing A/O. ratio. Fig. 4(c, d) shows the effect of A/O on percentage extraction which was satisfied by Eq. (6).