- Email: [email protected]
Reactive Extraction of o-Aminophenol Using Trialkylphosphine Oxide1
Chinese J. Chem. Eng., 14(1) 46-50
(2006)
Reactive Extraction of o-Aminophenol Using TrialkylphosphineOxide* LI Deliang(+@g)**, LTU Xiaoqiang(34 )b $I$) and CUI Jiehu(&T&) Chemistq and Chemical Engineering College, Henan University, Kaifeng 475001, China
Abstract Extraction of o-aminophenol (OAP) using trialkylphosphine oxide (TRPO) was studied with different diluents. The neutral OAP was extracted using TWO under an equilibrium pH in the range of 6-7.5, and a maximum distribution coefficient occurred. It was confirmed that the pH value and the TWO concentration are the key factors that affect distribution coefficient. Nonpolar diluents could provide better extraction distribution coefficient for the extraction of OAP and the order is: kerosene >n-octanol xhlorofonn. Keywords reactive extraction, o-aminophenol,trialkylphosphine oxide, diluent
1 INTRODUCTION Reversible reactive extraction is a new separation technique that is composed of several chemical reactions between certain functional groups of the extracted species and the extractant. It is a useful method to recover the organic species from the effluent with high effectiveness and selectivity"]. This technique is now receiving increased attention. A number of successful researches have been done in the fields of extraction mechanism, chemical engineering separation, and organic wastewater processing. It was found from these numerous studies that the organic species separated from aqueous solutions through reactive extraction contain only one Lewis functional group, Lewis acid, or Lewis base, for example, carboxylic acid^[^-^', phenols[61, or organic amineL7]solutions. However, very few research works about the extraction of compounds with two functional groups based on Lewis acidic and/or Lewis base have been reported. Amino acid is an important biochemical compound. In the production process of amino acid, the cost of separation and purification alone occupied more than 80% of the total cost. To obtain a new and excellent separation method, Liu et al. studied the extraction characteristics of phenylalanine'81,L-isole~cine[~], and L-tryptophan"'], and discussed the effect of pH on the extraction distribution coefficient. All these works were based on a theoretical foundation for the separation and recovery of amino acid. Yoshinori et al.'"' investigated the extraction of amino acid chelated with Cu and Ni and got good experimental results. This work proved to be useful in the exploration of the new
extractant. Aminobenzene sulfonic acid, aminobenzoic acid and aminophenol are important intermediates of dyes and medicines. Li et al.[12'13] carried out the determination of the distribution coefficient of aminobenzene sulfonic acid solution by using Alamine 336 and Aliquat 336 in n-octanol, chloroform and benzene, respectively. Compared to Alamine 336, Aliquat 336 could be used in a wider pH range. Liu, Zhang et aZ.[14'15' investigated the extraction equilibrium and mechanism of o, p-aminobenzoic acid solution with trioctylamine (TOA), tributyl phosphate (TBP), and di(2-ethylhexyl)-phosphoric acid (D2EHPA) as extractant. Li et aZ."6' studied the effect of different diluents and pH on the distribution coefficient of p-aminophenol by using trialkylphosphine oxide (TRPO) as extractant. Based on this research work, Qin et al."" attempted a further study on the extraction of p-aminophenol by using the mixture of trialkylamine and D2EHPA as extractant with n-heptane as diluent. They found that there was a synergistic effect when extracting p-aminophenol with the mixed solvent. This method has broadened the application range of reactive extraction. The research works referred to above give us a foundation for the reactive extraction of diluted solution of organic compounds bearing both acid and basic functional groups, but the theory of the extraction rule is not yet perfected. There has not been a suitable extraction model like the extraction of carboxylic acids with TOA"], which can predict the reactive extraction characteristics of amphoteric organic compounds. o-Aminophenol (OAP),a white needle crystal, is
Received 2005-03-08, accepted 2005-1 1-20.
* Supported by the Educational Department of Henan Province (No.2004530001). ** To whom correspondence should be addressed. E-mail: [email protected]
Reactive Extraction of o-AminophenolUsing Trialkylphosphine Oxide
s typical amphoteric organic compound. It is commonly used in pesticides, printing, dye, and pharmaceutical industries as starting material. In this paper, the reactive extraction characteristics of OAP with T W O were studied in n-octanol, kerosene, and chloroform diluents. The effects of T W O concentration and pH on the distribution coefficient were also investigated. These results will provide useful knowledge and experience for further investigation of OAP wastewater and the extraction of organic wastewater containing other amphoteric compounds with TRPO.
2 EXPERIMENTAL 2.1 Chemicals 0-Aminophenol was purchased from FLUKA (German) with purity 399% (by mass). It is not stable in an aqueous solution, so 0.1250g of OAP was dissolved in 50Oml H2SO4 solution with the concentration of 0.05mol.L-' ; The concentration of the aqueous OAP solution was 2.29 X 10-3m01.L-' (250mgL'). Kerosene was obtained from a local chemical plant. After a fractional distillation, the collected amount was in the range of 180°C to 220°C. The distilled kerosene was washed with concentrated sulfuric acid (98%) (VH,SO, Vkerosene =1 5 ) twice and distilled water several times until the aqueous layer became neutral. The density of the sulfonated kerosene was about 0 . 7 8 g ~ m - ~ . TRPO was from CYTEC Canada Incorporation with purity 393% (by mass) m e average molecular weight of TRPO was 350, the density was 0 . 8 8 g ~ m - ~ (25"C)I.TRPO contains a little acid and it will affect the pH of the system greatly. So it needs to be washed with a base before use. The TRPO solution was washed with 5% NaOH solution (VNaoH : Vm=l : 5), then diluted with HzSO4 (3%), and finally washed with distilled water until the aqueous layer became neutral. All other reagents were from the Beijing Chemical Reagent Plant with purity >99 % (by mass).
2.2 Experiments All extraction experiments were conducted with 1OOml conical flasks at 25 "C. Unless otherwise noted, 25ml of OAP solution and 25ml of the mixture (different concentration of diluents and TWO) were added to each conical flask. Saturated NaOH and diluted H2SO4 were used to adjust the pH of the system. The reaction mixture was shaken for about 1.5h in a constant temperature shaker bath with a vibrating rate of 180r-min-' and then was left to equilibrate for 1-2h;
47
finally two phases were available and separated. The pH value of the aqueous phase was measured with a pH meter (Hana pH HI 9321 model with a deviation of fO.O1, Italy) and then the pH of aqueous phase was adjusted to 1-1.5 by concentrated H2S04. OAP concentrations in the aqueous layers were analyzed at 271nm with a UV-250 spectrometer (Shimadzu, Japan). The solution concentrations in the organic phase were calculated by a material balance. The deviation of this method was less than 2%[17'.
3 RESULTS AND DISCUSSION OAP has one Lewis acid group, - O H , and one Lewis base group, -NH2. This amphoteric compound exists in three forms in aqueous solution: W3NArOH (A?, HzNArOH (A) and HzNArO- (A-). Two dissociation equilibriaexist in aqueous solutions as follows: K NHiArOH NH,ArOH+H+ (1)
-
NH,ArOH
K 82\ NH2ArO- + H+
(2)
with the dissociation balance constants of (1) and (2) (mo1.L-l): (3)
(4) where pKal and pKa2are 4.72 and 9.72, respectively['81. As can be seen from the above equations the pH of the solution affects the existing forms of the solute greatly. The cation of OAP exists at low pH (pHpKd), and neutral OAP dominates at an intermediate pH (pKal< pH
3.1 The effect of pH on the distributioncoefficient A TRPO (R3P=O) molecule has a phosphinyl and a lone-pair electron, so it is a Lewis base. TRPO reacts with neutral OAP by forming a hydrogen bond with hydroxyl of OAP. So, the molar fraction of neutral OAP will reach the maximum with the increase of the pH, and the distribution coefficient (0)also greatly depends on the pH of the aqueous phase. As Chinese J. Ch. E. 14(1)46(2006)
Chinese J. Ch. E. (Vol. 14, No.1)
48
indicated in Figs.2-4, the D value increased with the increase of the pH value and then decreased. There is a maximum D when the equilibrium pH value is between 6 and 7.5(initial pH value is between 6.5 and 8).0bviously, the variation trend of molar fraction (Fig.1) has the same order as that of the D value. The bigger the concentration of T W O is, the more evident is the departure of maximum D. As TFUO ' contains a little phosphorous acid"'] (R2POH) and aliphatic acidtzo1, its equilibrium solution pH tends to drop 0.5-2pH units compared with the initial pH. According to reference"'], this kind of acid cannot be eliminated by base washing completely. So, the acid will enter the aqueous phase and decrease the pH value in the aqueous phase in the extraction process. To avoid this condition, the purity of T W O and the optimization of pH operation should be considered before extraction.
Q20tQ 10
0'
3.0
I
I
I
1
4.0
6.0
5.0
7.0
8.0
PH,
Figure 3 Extraction behavior of OAP with TWOtn-0ctanol TWO concentration, rno1.L-l: W 0.266; 0 0.531; A 0.797; v 1.329; 2.338
+
-e=
I 3
4
5
6
7
8
9
PH,,
-2
0
I
I
I
I
I
I
2
4
6
8
10
12
+
I
14
16
PH Figure 1 Dependence of the fraction of o-aminophenol styles on pH A' - o-aminophenol cation; A- neutral o-aminophenol; A- - o-aminophenol anion
Figure 2 Extraction behavior of OAP with TRPO+kerosene TWO concentration, rno1-L-l: W 0.266; 0 0.531; A 0.797; v 1.329; 2.338
+
February, 2006
Figure 4 Extraction behavior of OAP with TRPO+chloroform TRPO concentration, mo1.L-l: 0.266; 0 0.531; A 0.797; v 1.329; 2.338
The effect of extractant concentration on the distribution coefficient
3.2
It can be seen b m F i g s . 2 4 that D values increased with the extractant concentrations.The increase of extractant concentration results in the increase of extractability. Consequently, the extraction equilibrium moves toward the direction of forming an extraction complex with the increase of extractant concentration.
3.3 The effect of diluents on the distributioncoeflicient From Fig.5, we can see that the physical extractability of n-octanol is greater than that of kerosene and chloroform. From Figs.2-4 and Fig.6, it can be seen that when the TRPO concentrations are the same, TRPOkerosene mixture has higher D values than that of TRPOln-octanol and TRPO/chloroform. This phenomenon is closely related to the following factors.
Reactive Extraction of o-AminophenolUsing Trialkylphosphine Oxide
This is owing to the fact that the acidity of OAP (pKa= 4.74, pKb4.74) is stronger than that of p-aminophenol (pKa=5.29, pKb=3.70). This result conforms to the theory of Lewis acid-base, which is the basis of reactive extraction.
-
3.0
3
4
5
7
6
8
9
10
PH.,
Figure 5 Extraction behavior of OAP with diluent W kerosene; A chloroform; n-octanol
+
25
t
49
n
PH,,
Figure 6 Extraction behavior of OAP with different diluent W 0.797molK’ TRPO -k kerosene; .0.797mo~~-’ TWO+ n-octanol; AO.797mol.L-’ TRPO+ chloroform
Chloroform has a stronger acidity than that of n-octanol and n-octanol in turn has a stronger acidity than that of kerosene. The hydrogen bonds formed between TRPO and acid diluents (chloroform, n-octanol) weaken the extractability of TRPO to O M . So the extractability of TRPO /chloroform or TRPOln-octanol is weaker than that of TRPO/ kerosene. ‘4 COMPARISON WITH OTHER WORKS We reported the extraction equilibrium of p-aminophenol with TRPO”61.As a little remnant phosphorous acid (RzPOH) and aliphatic acid in TRPO were not treated with alkali,it caused the emulsification phenomenon and therefore the peak value of extraction was not obtained. In our present work, two systems, 20% TRPO+PAP(p-aminophenol)+n-octanol(previous work) and 0.531mol.L-’+PAP+n-octanol (present work), are compared and it was found that the extraction capability of TRPO is greater than that of PAP (both have the same initial concentration of TRPO and the peak value D).
5 CONCLUSIONS (1) TRPO mainly reacts with neutral OAP. The distribution coefficient increased with the increase of undissociated neutral OAP in the aqueous phase. As TRPO contains a little phosphorous acid and aliphatic acid, the equilibrium solution pH is different from the initial one. The distribution coefficient greatly depends on the pH of the aqueous phase. (2) The experimental results show that polar diluents are not favorable to the reactive extraction of OAP with TRPO. Nonpolar diluents could provide better extraction distribution coefficient and the order is: kerosene > n-octanolxhloroform. The hydrogen bonds formed between TRPO and acid diluents weakened the extractability of TRPO to OAP. Kerosene will be a suitable choice as diluent when TRPO is used to extract OAP in wastewater.
NOMENCLATURE distribution Coefficient Kal the first dissociation constant of OAP, mol.L-’ Ka2 the second dissociation constant of OAF’, mo1.L-l
D
REFERENCES King, C.J., “Separation process based on reversible chemical complexation ” , Handbook of Separation Process Technology, ChapterlS, John Wiley & Sons, New York, 760-774 (1987). Li, Z.Y., “Extraction mechanism of carboxylic acid by tri-n-octylamine”, Ph. D. Thesis, Tsinghua Univ.,Beijing (2001). (in Chinese) Hong, Y.K., Hong, W.H., “Reactive extraction of succinic acid with tripropylamine (TPA) in various diluents”, Bioprocess Engineering, (22), 281-284(2OOO). Maria, M., George, K., J&l, A., Jacques, M., Guy, M.,”Separation of tartaric and lactic acids by means of solvent extraction”, Separation and Purification Technology, (37), 19!+-207(2004). Aynur, S., “Extraction equilibria of valeric acids using Alamine 308/diluent and conventional solvent systems. Modeling considerations”, Chem. Eng. Proc., (41), 681-692(2002). Li,G Z., Zhao, H., “Study on extracton of odhydmxybenzene with tributyl Phosphate”, Shanghai Environmental Sciences, 22(3), 197-199(2003). (in Chinese) Cai, R., Guan, GF., “Study on caprolactam waste water
Chinese J. Ch. E. 14(1)46(2006)
50
8
9
10
11
12
13
Chinese J. Ch. E. (Vol. 14, No.1) treatment”, Journal of Nanjing University of Chemical Technology,22(5), 73-75(2000). (in Chinese) Liu, Y. Sh., Dai, Y.Y., Wang, J.D., “Distribution behavior of L-phenylalanine by extraction with di(2-ethylhexyl) phosphoric acid”, Sep. Sci. Tech., 34(11), 21652176(1999). Liu, Y. Sh., Zhang, J., Dai, Y.Y., “Study on extraction of L-isoleucine with di(2-ethylhexy1)phosphoric acid”, J. Chem. Eng. Chin. Univ., 14(5), 415-419(2000). (in Chinese) Liu, Y.Sh., Di, Y.Y., “Extraction of L-tryptophane with di(2-ethylhexy1)phosphoric acid”, J. Chem. Ind. Eng. (China), 52(3), 216-221(2001). (in Chinese) Yoshinori, I., Kurose, S., Takashi, K., “Extraction of unprotected amino acids by mixed-ligand nickl (11) and copper (11) chelates”, Monatshefe fUr Chemie, 132, 1433-1438(200 1). Li, Zh. Y., Yang, Y.Y., Dai, Y.Y., “Extraction of p-amionbenzen sulfonic acid with trialkylamine(7301) based on chemical complexation”, J. Chem. Ind. Eng. (China), 51(1), 85-89(2000). (in Chinese) Li, Zh.Y., Qin, W., Yang, Y.Y., Dai, Y.Y., “Extraction p-aminobenzene sulfonic acid from dilute solution by chemical complexation with aliquat 336”, J. Chem. Eng. Chin. Univ., 15(6), 578-582(2001). (in Chinese)
14
15
16
17
18 19
20
Zhang, J., Liu, Y. Sh., Liu, Z;Y., Dai, Y.Y., “A study on extraction aminobenzoic acid based on chemical complexation”, Environmental Chemistry, 19(2), 131 135(2000). (in Chinese) Zhang, J., Dai, Y.Y., “Distribution behavior of aminobenzoic acid by extraction with di(2-ethylhexy1)phosphoric acid”, Chinese J. Chem. Eng., 8(4), 300303(2000). Li, D.L., Qin, W., Dai, Y.Y., “Extraction equilibrium of p-aminophenol with TWO’, J. Chem. Znd. Eng. (China), 54(3), 339-342(2003). (in Chinese) Qin, W., Li, D.L., Dai, Y.Y., “Liquid-liquid Equilibria of p-aminophenol between water and trimlamine, t r i m l phosphine oxide, and di(2-ethylhexy1)phosphoric acid in heptane”, J. Chem Eng. Data, 48,16061609(2003). Dean, J.A., Lang’s Handbook of Chemistry, McGraw-Hill, New York (1985). Lu, X.Y., “Synthesis of phosphoric extractant”, Science and Technology of Nuclear Energy, (6), 627-635( 1964). (in Chinese) Xin, R.X., Wu, W., Jiao, J.X., “Determination of trace fatty acids in tri-alkyphosphine oxide by pyrolysis methylation gas chromatography”, Chinese Journal of Analysis Laboratory, 20( l), 8-ll(2001). (in Chinese)
(2006)
Reactive Extraction of o-Aminophenol Using TrialkylphosphineOxide* LI Deliang(+@g)**, LTU Xiaoqiang(34 )b $I$) and CUI Jiehu(&T&) Chemistq and Chemical Engineering College, Henan University, Kaifeng 475001, China
Abstract Extraction of o-aminophenol (OAP) using trialkylphosphine oxide (TRPO) was studied with different diluents. The neutral OAP was extracted using TWO under an equilibrium pH in the range of 6-7.5, and a maximum distribution coefficient occurred. It was confirmed that the pH value and the TWO concentration are the key factors that affect distribution coefficient. Nonpolar diluents could provide better extraction distribution coefficient for the extraction of OAP and the order is: kerosene >n-octanol xhlorofonn. Keywords reactive extraction, o-aminophenol,trialkylphosphine oxide, diluent
1 INTRODUCTION Reversible reactive extraction is a new separation technique that is composed of several chemical reactions between certain functional groups of the extracted species and the extractant. It is a useful method to recover the organic species from the effluent with high effectiveness and selectivity"]. This technique is now receiving increased attention. A number of successful researches have been done in the fields of extraction mechanism, chemical engineering separation, and organic wastewater processing. It was found from these numerous studies that the organic species separated from aqueous solutions through reactive extraction contain only one Lewis functional group, Lewis acid, or Lewis base, for example, carboxylic acid^[^-^', phenols[61, or organic amineL7]solutions. However, very few research works about the extraction of compounds with two functional groups based on Lewis acidic and/or Lewis base have been reported. Amino acid is an important biochemical compound. In the production process of amino acid, the cost of separation and purification alone occupied more than 80% of the total cost. To obtain a new and excellent separation method, Liu et al. studied the extraction characteristics of phenylalanine'81,L-isole~cine[~], and L-tryptophan"'], and discussed the effect of pH on the extraction distribution coefficient. All these works were based on a theoretical foundation for the separation and recovery of amino acid. Yoshinori et al.'"' investigated the extraction of amino acid chelated with Cu and Ni and got good experimental results. This work proved to be useful in the exploration of the new
extractant. Aminobenzene sulfonic acid, aminobenzoic acid and aminophenol are important intermediates of dyes and medicines. Li et al.[12'13] carried out the determination of the distribution coefficient of aminobenzene sulfonic acid solution by using Alamine 336 and Aliquat 336 in n-octanol, chloroform and benzene, respectively. Compared to Alamine 336, Aliquat 336 could be used in a wider pH range. Liu, Zhang et aZ.[14'15' investigated the extraction equilibrium and mechanism of o, p-aminobenzoic acid solution with trioctylamine (TOA), tributyl phosphate (TBP), and di(2-ethylhexyl)-phosphoric acid (D2EHPA) as extractant. Li et aZ."6' studied the effect of different diluents and pH on the distribution coefficient of p-aminophenol by using trialkylphosphine oxide (TRPO) as extractant. Based on this research work, Qin et al."" attempted a further study on the extraction of p-aminophenol by using the mixture of trialkylamine and D2EHPA as extractant with n-heptane as diluent. They found that there was a synergistic effect when extracting p-aminophenol with the mixed solvent. This method has broadened the application range of reactive extraction. The research works referred to above give us a foundation for the reactive extraction of diluted solution of organic compounds bearing both acid and basic functional groups, but the theory of the extraction rule is not yet perfected. There has not been a suitable extraction model like the extraction of carboxylic acids with TOA"], which can predict the reactive extraction characteristics of amphoteric organic compounds. o-Aminophenol (OAP),a white needle crystal, is
Received 2005-03-08, accepted 2005-1 1-20.
* Supported by the Educational Department of Henan Province (No.2004530001). ** To whom correspondence should be addressed. E-mail: [email protected]
Reactive Extraction of o-AminophenolUsing Trialkylphosphine Oxide
s typical amphoteric organic compound. It is commonly used in pesticides, printing, dye, and pharmaceutical industries as starting material. In this paper, the reactive extraction characteristics of OAP with T W O were studied in n-octanol, kerosene, and chloroform diluents. The effects of T W O concentration and pH on the distribution coefficient were also investigated. These results will provide useful knowledge and experience for further investigation of OAP wastewater and the extraction of organic wastewater containing other amphoteric compounds with TRPO.
2 EXPERIMENTAL 2.1 Chemicals 0-Aminophenol was purchased from FLUKA (German) with purity 399% (by mass). It is not stable in an aqueous solution, so 0.1250g of OAP was dissolved in 50Oml H2SO4 solution with the concentration of 0.05mol.L-' ; The concentration of the aqueous OAP solution was 2.29 X 10-3m01.L-' (250mgL'). Kerosene was obtained from a local chemical plant. After a fractional distillation, the collected amount was in the range of 180°C to 220°C. The distilled kerosene was washed with concentrated sulfuric acid (98%) (VH,SO, Vkerosene =1 5 ) twice and distilled water several times until the aqueous layer became neutral. The density of the sulfonated kerosene was about 0 . 7 8 g ~ m - ~ . TRPO was from CYTEC Canada Incorporation with purity 393% (by mass) m e average molecular weight of TRPO was 350, the density was 0 . 8 8 g ~ m - ~ (25"C)I.TRPO contains a little acid and it will affect the pH of the system greatly. So it needs to be washed with a base before use. The TRPO solution was washed with 5% NaOH solution (VNaoH : Vm=l : 5), then diluted with HzSO4 (3%), and finally washed with distilled water until the aqueous layer became neutral. All other reagents were from the Beijing Chemical Reagent Plant with purity >99 % (by mass).
2.2 Experiments All extraction experiments were conducted with 1OOml conical flasks at 25 "C. Unless otherwise noted, 25ml of OAP solution and 25ml of the mixture (different concentration of diluents and TWO) were added to each conical flask. Saturated NaOH and diluted H2SO4 were used to adjust the pH of the system. The reaction mixture was shaken for about 1.5h in a constant temperature shaker bath with a vibrating rate of 180r-min-' and then was left to equilibrate for 1-2h;
47
finally two phases were available and separated. The pH value of the aqueous phase was measured with a pH meter (Hana pH HI 9321 model with a deviation of fO.O1, Italy) and then the pH of aqueous phase was adjusted to 1-1.5 by concentrated H2S04. OAP concentrations in the aqueous layers were analyzed at 271nm with a UV-250 spectrometer (Shimadzu, Japan). The solution concentrations in the organic phase were calculated by a material balance. The deviation of this method was less than 2%[17'.
3 RESULTS AND DISCUSSION OAP has one Lewis acid group, - O H , and one Lewis base group, -NH2. This amphoteric compound exists in three forms in aqueous solution: W3NArOH (A?, HzNArOH (A) and HzNArO- (A-). Two dissociation equilibriaexist in aqueous solutions as follows: K NHiArOH NH,ArOH+H+ (1)
-
NH,ArOH
K 82\ NH2ArO- + H+
(2)
with the dissociation balance constants of (1) and (2) (mo1.L-l): (3)
(4) where pKal and pKa2are 4.72 and 9.72, respectively['81. As can be seen from the above equations the pH of the solution affects the existing forms of the solute greatly. The cation of OAP exists at low pH (pH
3.1 The effect of pH on the distributioncoefficient A TRPO (R3P=O) molecule has a phosphinyl and a lone-pair electron, so it is a Lewis base. TRPO reacts with neutral OAP by forming a hydrogen bond with hydroxyl of OAP. So, the molar fraction of neutral OAP will reach the maximum with the increase of the pH, and the distribution coefficient (0)also greatly depends on the pH of the aqueous phase. As Chinese J. Ch. E. 14(1)46(2006)
Chinese J. Ch. E. (Vol. 14, No.1)
48
indicated in Figs.2-4, the D value increased with the increase of the pH value and then decreased. There is a maximum D when the equilibrium pH value is between 6 and 7.5(initial pH value is between 6.5 and 8).0bviously, the variation trend of molar fraction (Fig.1) has the same order as that of the D value. The bigger the concentration of T W O is, the more evident is the departure of maximum D. As TFUO ' contains a little phosphorous acid"'] (R2POH) and aliphatic acidtzo1, its equilibrium solution pH tends to drop 0.5-2pH units compared with the initial pH. According to reference"'], this kind of acid cannot be eliminated by base washing completely. So, the acid will enter the aqueous phase and decrease the pH value in the aqueous phase in the extraction process. To avoid this condition, the purity of T W O and the optimization of pH operation should be considered before extraction.
Q20tQ 10
0'
3.0
I
I
I
1
4.0
6.0
5.0
7.0
8.0
PH,
Figure 3 Extraction behavior of OAP with TWOtn-0ctanol TWO concentration, rno1.L-l: W 0.266; 0 0.531; A 0.797; v 1.329; 2.338
+
-e=
I 3
4
5
6
7
8
9
PH,,
-2
0
I
I
I
I
I
I
2
4
6
8
10
12
+
I
14
16
PH Figure 1 Dependence of the fraction of o-aminophenol styles on pH A' - o-aminophenol cation; A- neutral o-aminophenol; A- - o-aminophenol anion
Figure 2 Extraction behavior of OAP with TRPO+kerosene TWO concentration, rno1-L-l: W 0.266; 0 0.531; A 0.797; v 1.329; 2.338
+
February, 2006
Figure 4 Extraction behavior of OAP with TRPO+chloroform TRPO concentration, mo1.L-l: 0.266; 0 0.531; A 0.797; v 1.329; 2.338
The effect of extractant concentration on the distribution coefficient
3.2
It can be seen b m F i g s . 2 4 that D values increased with the extractant concentrations.The increase of extractant concentration results in the increase of extractability. Consequently, the extraction equilibrium moves toward the direction of forming an extraction complex with the increase of extractant concentration.
3.3 The effect of diluents on the distributioncoeflicient From Fig.5, we can see that the physical extractability of n-octanol is greater than that of kerosene and chloroform. From Figs.2-4 and Fig.6, it can be seen that when the TRPO concentrations are the same, TRPOkerosene mixture has higher D values than that of TRPOln-octanol and TRPO/chloroform. This phenomenon is closely related to the following factors.
Reactive Extraction of o-AminophenolUsing Trialkylphosphine Oxide
This is owing to the fact that the acidity of OAP (pKa= 4.74, pKb4.74) is stronger than that of p-aminophenol (pKa=5.29, pKb=3.70). This result conforms to the theory of Lewis acid-base, which is the basis of reactive extraction.
-
3.0
3
4
5
7
6
8
9
10
PH.,
Figure 5 Extraction behavior of OAP with diluent W kerosene; A chloroform; n-octanol
+
25
t
49
n
PH,,
Figure 6 Extraction behavior of OAP with different diluent W 0.797molK’ TRPO -k kerosene; .0.797mo~~-’ TWO+ n-octanol; AO.797mol.L-’ TRPO+ chloroform
Chloroform has a stronger acidity than that of n-octanol and n-octanol in turn has a stronger acidity than that of kerosene. The hydrogen bonds formed between TRPO and acid diluents (chloroform, n-octanol) weaken the extractability of TRPO to O M . So the extractability of TRPO /chloroform or TRPOln-octanol is weaker than that of TRPO/ kerosene. ‘4 COMPARISON WITH OTHER WORKS We reported the extraction equilibrium of p-aminophenol with TRPO”61.As a little remnant phosphorous acid (RzPOH) and aliphatic acid in TRPO were not treated with alkali,it caused the emulsification phenomenon and therefore the peak value of extraction was not obtained. In our present work, two systems, 20% TRPO+PAP(p-aminophenol)+n-octanol(previous work) and 0.531mol.L-’+PAP+n-octanol (present work), are compared and it was found that the extraction capability of TRPO is greater than that of PAP (both have the same initial concentration of TRPO and the peak value D).
5 CONCLUSIONS (1) TRPO mainly reacts with neutral OAP. The distribution coefficient increased with the increase of undissociated neutral OAP in the aqueous phase. As TRPO contains a little phosphorous acid and aliphatic acid, the equilibrium solution pH is different from the initial one. The distribution coefficient greatly depends on the pH of the aqueous phase. (2) The experimental results show that polar diluents are not favorable to the reactive extraction of OAP with TRPO. Nonpolar diluents could provide better extraction distribution coefficient and the order is: kerosene > n-octanolxhloroform. The hydrogen bonds formed between TRPO and acid diluents weakened the extractability of TRPO to OAP. Kerosene will be a suitable choice as diluent when TRPO is used to extract OAP in wastewater.
NOMENCLATURE distribution Coefficient Kal the first dissociation constant of OAP, mol.L-’ Ka2 the second dissociation constant of OAF’, mo1.L-l
D
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