Various techniques remove Cu(II) from aqueous solutions, membrane filtration, flotation, electrolysis, biosorption, precipitation [4-5] and liquid- liquid extraction. Between various techniques extract Cu(II) from aqueous solutions, Liquid-liquid extraction is one of the effective techniques to extract Cu(II) from aqueous solutions [6]. So, liquid- liquid extraction is established technologic for recovery of metals from dilute aqueous product effluents [7-12]. In this work, Investigation reports on use solvent extraction in the separation of Cu(II) from aqueous solutions by lauric acid diluted in benzene under different experimental conditions: aqueous pH, Cu(II) and extractant concentration, and extract.

Materials

All the chemicals were used without further purification. Copper sulphate pentahydrate (CuSO₄.5H₂O) (Merck ≥ 99.6% purity), benzene (Merck ≥ 99% purity), lauric acid (probus ≥ 99% purity) sulphuric acid (H₂SO₄) (Probus R.A ≥ 98% purity) were purchased. Distilled and deionized water was used throughout all experiments. The structure of lauric acid is shown in I (Structure 1).

Extraction procedures

A volume of 10 ml of Cu(II) were prepared by dissolving appropriate amounts of CuSO₄.5H₂O (5 X 10^-4, 2.5 X 10^-3, 5X 10^-3 and 2.5 X 10^-2) M in distilled water loaded with 0.1M Na2SO4 and containing aqueous phase at 1:1 organic to aqueous volume ratio in glass cell. Organic phase was prepared with lauric acid (extractant) and benzene (diluents). A glass cell connected to a water thermostat was made to measure the liquid-liquid extraction data. The prepared mixtures were introduced into the extraction cell and were stirred for 2 h, and then left to settle for 2 h for phase separation. After being allowed to reach equilibrium, samples were carefully taken from each phase. Aqueous phase pH was measured with a Radiometer Copenhagen pH meter (model 62). The samples was used to 1.8 g/dm³ H₂SO₄, pH=1.6. The concentration of the Cu (II) in organic phase was obtained from a Hitachi UV-Vis Spectrophotometer (model 40-100) at a wavelength of 412 nm. All the experiments were carried out at constant temperature T=298.2 K. The temperature was estimated to be accurate to within solution ± 0.1K. The percentage extractions (%E) of Cu(II) were calculated according to [7-8]:

%E=(1)

where [Cu]_initial,aq is the initial Cu(II) concentration in the aqueous phase and [Cu]_r,aq is the remaining Cu(II) concentration in the aqueous phase after extraction.

Theory

The extraction process may be represented by the equation,

(2)

where represents the extraction reagent. The extraction constant of the species is given by:

(3)

where is the only extractable species. Introducing the mass balance equation for the Cu(II)and the extractant

(4)

The mass balance equation for the Cu(II) in the aqueous phase

[Cu]= [Cu²⁺] + [CuR⁺] + [CuR₂] + [CuSO₄]       (5) 

[Cu]=  [Cu²⁺](6)

where is formation constant of Cu(II). The extraction constant of the total Copper in the aqueous phase is given by

 = (7)

The metal distribution ratio (D’) and the extraction constant are related by

(8)

(9)

According to (Equation 9) a plot of logD’ versus pH will give straight line shown in Figures 1, 2. The distribution coefficients (D’) increase with increasing pH. A plot of log D’ – log [Cu] against pH (Figure 3) will give a straight line of slope and intercept log K_ex at constant concentration of log in Table 1. The Figure 4 log D’ –log [Cu] - 4pH versus of log will show a straight line of slope and intercept log K_ex. Parameter correlation equilibrium data for extraction of Cu(II) using lauric acid diluted in benzene at 298.2 K presented in Table 2. The extraction constant (log K_ex) has been calculated as -12.2580.

Figure 1: The plot of distribution coefficient (log D’) against pH at the extraction of Cu (I) with lauric acid (0.2 M) diluted in benzene at 298.2 K. Initial concentration of Cu(I) (M): (●) 5×10^-4; (○) 2.5×10^-3; (Δ) 5×10^-3; (▲) 2.5×10^-2.

Figure 2: The plot of distribution coefficient (log D’) against pH at the extraction of Cu(I) with lauric acid (0.5 M) in benzene at 298.2 K. Initial concentration of Cu(I) (M): (●) 5×10-4; (○) 2.5×10-3; (Δ) 5×10-3; (▲) 2.5×10-2.

Figure 3: The plots of log D’ –log [Cu] versus pH at the extraction of Cu(I) with lauric acid diluted in benzene at 298.2 K. Initial concentration of Cu(I) (M): concentration of 0.2 M lauric acid: (●) 5×10^-4; (○) 2.5×10^-3; (Δ) 5×10^-3; (▲) 2.5×10^-2 and concentration of 0.5 M lauric acid: (■) 5×10^-4; (□) 2.5×10^-3; (×) 5×10^-3; (◊) 2.5×10^-2 .

Figure 4: The plots of log D’ – log [Cu] - 4pH against log [ H₂R₂ ] of the extraction of Cu(I) with lauric acid diluted in benzene at 298.2 K. Initial concentration of Cu(I) (M): (●) 5×10^-4; (○) 2.5×10^-3; (Δ) 5×10^-3; (▲) 2.5×10^-2.

×10-3 (mol-g/dm3)	×10-3 (mol-g/dm3)	D'	pH	(mol-g/dm3)
1.88	3.12	0.60	4.52	0.05
1.89	3.11	0.61	4.30	0.10
1.91	3.09	0.62	4.18	0.15
1.99	3.01	0.66	4.05	0.25
2.05	2.95	0.70	3.91	0.35
2.09	2.91	0.72	3.80	0.50

Table 1: Equilibrium data of (CuSO₄ + lauric acid + benzene) at 298.2 K: influence of luaric acid concentration. C_cu,initial = 5×10^-3 M , CNa₂SO₄ = 0.1 M (14.204 g/dm³).

Table 2: Parameter correlation equilibrium data for extraction of Cu(II) with using lauric acid diluted in benzene at 298.2 K.

Data extraction equilibrium for initial Cu (II) concentration (5 X 10^-4, 2.5 X 10^-3, 5X 10^-3 and 2.5 X 10^-2 ) M in pH=1.6 Sulphuric acid solution in the range of 0.2 - 0.5 M Lauric acid, pH for extraction, distribution coefficient (D’), percentage extraction (%E) given in Tables 3-6 at atmospheric pressure and at T=298.2 K. Table 1 gives influence of lauric acid concentration of the equilibrium data of CuSO₄ and lauric acid diluted in benzene.

Cacid = 0.2 M (40.07 g/dm3)						Cacid = 0.5  M (100.17 g/dm3)
10-4 (mol-g /dm3 )	[Cu]×10-4 (mol-g /dm3)	D’	%E	pH		10-4 ( mol-g /dm3 )	[Cu]×10-3 (mol-g /dm3)	D’	%E		pH
0.866	4.14	0.21	17.3	4.56		0.775	4.23	0.18		15.5	4.17
1.15	3.86	0.30	22.9	4.59		1.07	3.93	0.27		21.5	4.22
1.58	3.42	0.46	31.6	4.67		1.70	3.30	0.52		34.1	4.30
2.01	2.99	0.67	40.1	4.70		1.98	3.06	0.65		39.6	4.34
2.21	2.80	0.79	44.1	4.74		2.55	2.45	1.04		51.0	4.42
2.27	2.73	0.83	45.4	4.76		3.87	1.13	3.43		77.4	4.63
2.96	2.04	1.46	59.3	4.86		4.24	0.759	5.59		84.8	4.72
3.72	1.28	2.90	74.4	4.96		4.62	0.386	12.0		92.3	4.88
4.03	0.975	4.13	80.5	5.03		4.72	0.284	16.6		94.3	4.95
4.86	0.147	33.1	97.1	5.47		4.88	0.119	40.9		97.6	5.16

Table 3: Equilibrium data of (CuSO₄ + lauric acid + benzene) at 298.2 K. C_cu,initial = 5×10^-4 M (0.0318 g/dm³) , CNa₂SO₄ = 0.1 M (14.204 g/dm³).

Cacid = 0.2 M (40.07 g/dm3)					Cacid = 0.5 M (100.17 g/dm3)
(mol-g /dm3 )	( mol-g /dm3 )	D'	%E	pH	(mol-g /dm3 )	( mol-g /dm3 )	D'	%E	pH
0.310	2.19	0.14	12.4	4.26	0.171	2.33	0.07	6.8	3.83
0.454	2.05	0.22	18.2	4.31	0.419	2.08	0.20	16.8	3.95
0.638	1.86	0.34	25.5	4.36	0.790	1.71	0.32	31.6	4.02
0.691	1.81	0.38	27.6	4.39	0.945	1.56	0.61	37.8	4.10
0.974	1.53	0.64	39.0	4.46	0.122	1.28	0.96	48.9	4.17
1.03	1.47	0.89	41.2	4.50	0.172	0.784	2.19	68.7	4.31
1.40	1.10	1.28	56.1	4.56	0.191	0.587	3.26	76.5	4.38
1.85	6.52	2.83	73.9	4.71	2.05	0.447	4.60	82.1	4.45
2.25	2.52	8.91	89.9	4.94	2.09	0.412	5.07	83.5	4.47
2.48	0.02	123	99.2	5.54	2.15	0.353	6.08	85.9	4.51

Table 4: Equilibrium data of (CuSO₄ + lauric acid + benzene) at 298.2 K. C_cu,initial = 2.5×10^-3 M (0.1589 g/dm³) , CNa₂SO₄ = 0.1 M (14.204 g/dm³).

Cacid = 0.2 M (40.07 g/dm3)						Cacid = 0.5 M (100.17 g/dm3)
(mol /dm3 )	( mol /dm3 )	D'	%E	pH		(mol /dm3 )	( mol /dm3 )	D'	%E	pH
0.309	4.69	0.07	6.2	4.05		0.148	4.85	0.03	3.0	3.57
0.422	4.58	0.09	8.4	4.08		0.293	4.71	0.06	5.9	3.65
0.603	4.40	0.14	12.1	4.12		0.463	4.54	0.10	9.3	3.71
1.35	3.65	0.37	27.0	4.25		0.536	4.47	0.13	11.3	3.74
2.35	2.66	0.88	47.0	4.38		1.47	3.53	0.42	29.4	3.89
2.40	2.60	0.92	48.0	4.39		1.89	3.11	0.61	37.8	3.94
2.49	2.51	0.99	49.8	4.40		2.10	2.91	0.72	42.0	3.97
3.12	1.88	1.66	62.4	4.48		2.55	2.46	1.04	51.2	4.03
3.34	1.66	2.04	67.2	4.52		3.15	1.85	1.70	63.0	4.12
3.64	1.36	2.68	72.8	4.57		3.80	1.21	3.14	76.0	4.22
4.24	0.762	5.56	84.8	4.72		4.83	1.69	25.6	96.7	4.68
4.63	0.370	12.5	92.6	4.89		4.99	1.22	396	99.7	5.24

Table 5: Equilibrium data of (CuSO₄ + lauric acid + benzene) at 298.2 K. C_cu,initial = 5×10^-3 M (0.3177 g/dm³) , CNa₂SO₄ = 0.1 M (14.204 g/dm³).

Cacid = 0.2 M(40.07 g/dm3)					Cacid = 0.5 M(100.17 g/dm3)
(mol /dm3 )	( mol /dm3 )	D'	%E	pH	(mol /dm3 )		(mol /dm3 )	D'	%E	pH
0.067	2.43	0.03	2.7	3.86	0.205		2.23	0.09	8.2	3.46
0.10	2.40	0.04	4.0	3.88	0.393		2.11	0.19	15.7	3.55
0.132	2.37	0.06	5.3	3.89	0.467		2.03	0.23	18.7	3.58
0.136	2.36	0.06	5.4	3.90	0.629		1.87	0.34	25.2	3.63
0.150	2.35	0.06	6.0	3.91	0.735		1.77	0.42	29.4	3.66
0.199	2.30	0.09	8.0	3.93	0.889		1.61	0.55	35.5	3.70
0.216	2.28	0.09	8.6	3.96	1.20		1.30	0.92	48.0	3.78
0.381	2.12	0.18	15.2	3.98	1.51		0.996	1.51	60.2	3.86
0.724	1.78	0.41	29.0	4.09	1.74		0.763	2.28	69.5	3.93
0.934	1.57	0.60	37.4	4.15	1.82		0.676	2.70	73.0	3.96
1.10	1.41	0.78	43.8	4.20	1.95		0.551	3.54	78.0	4.02
1.41	1.09	1.29	56.4	4.31	2.01		0.493	4.08	80.4	4.04
1.59	0.908	1.80	63.7	4.36	2.10		0.401	5.24	84.0	4.09
1.69	0.807	2.10	67.6	4.40	2.23		0.274	8.12	89.0	4.18
1.89	0.614	3.07	75.4	4.49	2.47	0.033		73.9	98.7	4.65
2.35	0.152	15.4	93.9	4.81	0.25	0.003		874	99.5	5.19

Table 6: Equilibrium data of (CuSO₄ + lauric acid + benzene) at 298.2 K. C_cu,initial = 2.5×10^-2 M (1.5886 g/dm³) , CNa₂SO₄ = 0.1 M (14.204 g/dm³).

Effect of pH and Extractant concentration

Creature Cu (II) extraction by lauric acid depend on the initial acidity of the aqueous solution, more studied were carried out in order to recognize the influence of the aqueous pH on Cu (II) extraction. The pH will be very essential factor in the separation of metal ion. The results obtained were shown in Figs. 5 and 6, plotting the percentage of copper extraction (%E) against equilibrium pH. From Figures 5 and 6 find the lowest at pH of 3.46 and maximum pH of 5.54. The extraction percentage (%E) increased with growth of pH. It also can be seen that the curves are shifted to the left as the increase of initial Cu (II) concentration.

Figure 5: Effect of pH on the extraction of Cu (II) with lauric acid (0.2 M) diluted in benzene at 298.2 K. Initial concentration of Cu (II) (M): (●) 5×10^-4; (○) 2.5×10^-3; (Δ) 5×10^-3; (▲) 2.5×10^-2.

Figure 6: 6ct the extraction of Cu (II) with lauric acid (0.5 M) diluted in benzene at 298.2 K. Initial concentration of Cu II) (M): (●) 5×10^-4; (○) 2.5×10^-3; (Δ) 5×10^-3; (▲) 2.5×10^-2.

Thermodynamic part

Free energy (thermodynamic parameter) concerned in this study due to the transfer of a unit mole of Cu (II) from the aqueous into the organic phases. The of Cu (II) extraction at T=298.2 K was determined from [13]

(10)

Where R is the universal gas constant (8.314 J/mol K), T is the thermodynamic temperature and K_ex is the extraction constant and this was presented in Table 7 and the value is negative. The negative of indicated that the Cu (II) extraction with lauric acid occurred spontaneously at T=298.2 K. From log Kex versus log [Cu] (Figure 7) the extraction constant (log K_ex increased with increasing Cu (II) concentration in aqueous phase.

Figure 7: The plot of log Kex against [Cu] of the extraction of Copper (II) with lauric acid diluted in benzene at 298.2 K: Initial concentration of Cu(II) (M): (Δ) 5×10^-3.

10-3 (mol-g/dm3)	Kex	HR (kJ/mol)
3.12	9.248×10+23	-136.838
3.11	1.554×10+22	-126.707
3.09	1.560×10+21	-121.006
3.01	1.112×10+20	-114.457
2.95	1.208×10+19	-108.952
2.91	1.568×10+18	-103.890

Table 7: The extraction constant and free energy of equilibrium data for extraction of Cu (II) with using lauric acid diluted in benzene at 298.2 K.

The experimental data indicate lauric acid in organic phase extracts capably Cu(II) from aqueous solutions in the pH =1.6 Sulphuric acid at T=298.2 K under atmospheric pressure. The distribution coefficient (D’) and extraction percentage (%E) increased with growth of pH. The results indicate pH, percentage extraction (%E), and distribution coefficient (D’) decrease with increasing initial Cu(II) concentration (5 X 10^-4 to 2.5 X 10^-2) M and they raise with increasing lauric acid concentration 0.2 to 0.5 M. The negative of indicated that the Cu(II) extraction with lauric acid occurred spontaneously.