Extraction of phenols and phenyl acetates with diethyl carbonate

Analytica Chimica Acta 535 (2005) 251–257

Extraction of phenols and phenyl acetates with diethyl carbonate Jacek Olejniczak, Jacek Staniewski∗ , Jan Szymanowski Institute of Chemical Technology and Engineering, Pozna´n University of Technology, Pl. M. Skłodowska-Curie 2, 60-965 Pozna´n, Poland Received 22 September 2004; received in revised form 30 November 2004; accepted 30 November 2004 Available online 6 January 2005

Abstract Extraction of 11 selected phenols and their acetyl derivatives by diethyl carbonate was studied and compared with extraction into hexane and toluene. Diethyl carbonate is the most effective solvent and enables high recoveries of both phenols and their acetyl derivatives. The high recovery of phenols can be explained by the formation of a hydrogen bond between the hydroxyl group of phenols and the oxygen atom of the carbonate group. The extraction of phenols can be further increased in the presence of an electrolyte. The efficiency of extraction depends mainly upon phenol hydrophobicity and significantly increases with an increase in the solute molar volume and octanol–water partition coefficient. © 2004 Elsevier B.V. All rights reserved. Keywords: Solvent extraction; Phenols; Phenyl acetates; Abraham model; Diethyl carbonate; Capillary gas chromatography

1. Introduction Phenols are toxic pollutants frequently found in water surface and top waters. They are listed in the US Environmental Protection Agency priority list of pollutants and in the 76/464/EEC Directive of the European Union, related to hazardous substances discharged into the aquatic environments. Phenols are commonly encountered in aqueous effluents from various manufacturing processes [1]. Solvent extraction is the most often used technique to recover phenol [2]. In the past, toxic aromatic hydrocarbons were used for the recovery of phenols both in the laboratory and industrial scale. Currently, various ethers, including t-butyl methyl ether—an additive to gasoline, are mainly used. Reactive extraction with trialkylphosphine oxides and sulphides, di- and trialkylamines dissolved in high-boiling hydrocarbons are also used [3–8]. Nowadays, new environmentally friendly techniques such as micellar enhanced ultrafiltration or cloud point are used as well to recover phenols [9,10]. Dialkyl carbonates are not classified as dangerous or toxic substances according to the 2001/59/EC Directive of the Eu∗

Corresponding author. Tel.: +48 61 6653771; fax: +48 61 6653649. E-mail address: [email protected] (J. Staniewski).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.11.080

ropean Union. Due to their environmental friendly features, alkyl carbonates may be used in green synthesis [11,12]. In this way, hydrocarbons and organic reagents employed in classical extraction, which are dangerous for the environment, are eliminated. Extraction of phenols is also an important preconcentration step in their determination by gas chromatography (GC) and liquid chromatography (LC) [10,13–15]. To obtain high efficiencies in extraction and a good resolution of sharp peaks in GC, phenols are usually converted into less polar phenyl acetates. It was the aim of this work to study the possibility of using diethyl carbonate as a solvent for the effective extraction of 11 phenols having various substituents and their resulting various acid–base properties and hydrophobicities. Diethyl carbonate was selected as a model solvent, mainly from analytical consideration to simplify the simultaneous determination of 11 phenols and their acetyl derivatives. In technological applications, the use of more hydrophobic dialkyl or alkylene carbonates would be preferred. However, diethyl carbonate is the most hydrophobic of the currently manufactured dialkyl carbonates and alkylene carbonates. To-date, dialkyl carbonates have not been used for extraction of phenols and their acetyl derivatives. An additional aim of this work was to use

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the Abraham model [16–21] to correlate extraction results and determine the effect of phenol structure on extraction efficiency.

ones mentioned earlier but the initial oven temperature was held at 50 ◦ C. 2.3. Extraction procedure

2. Experimental 2.1. Materials The following phenols were used: phenol (99% purity), 4-nitrophenol (98% purity), 4-chloro-3-methylphenol (99% purity), each from POCh (Poland); 4-methylphenol (98% purity), from Merck (Germany); 3-chlorophenol (98% purity), 4-chlorophenol (99% purity), 2,3-dimethylphenol (98% purity), 2,4-dichlorophenol (99% purity), 4-methoxyphenol (99% purity), each from Sigma–Aldrich (Germany); 2,4,6trichlorophenol (99.9% purity), 2,3,4,6-tetrachlorophenol (99.9% purity), each from Fluka AG (Germany). A stock solution of all the phenols was prepared in acetonitrile (99.5% purity) from Fluka AG (Germany) and contained: phenol 990 ng ␮l−1 , 4-methylphenol 1018 ng ␮l−1 , 3chlorophenol 990 ng ␮l−1 , 4-chlorophenol 932 ng ␮l−1 , 2,3-dimethylphenol 916 ng ␮l−1 , 4-chloro-3-methylphenol 1046 ng ␮l−1 , 2,4-dichlorophenol 956 ng ␮l−1 , 2,4,6trichlorophenol 904 ng ␮l−1 , 4-nitrophenol 1020 ng ␮l−1 , 2,3,4,6-tetrachlorophenol 990 ng ␮l−1 and 4-methoxyphenol 1312 ng ␮l−1 . Acetic anhydride, pure for analysis, Fluka AG (Germany) was used for derivatization of phenols. Potassium carbonate, pure for analysis, from POCh (Poland), was used to adjust pH to 11. Hexane (99% purity), toluene, pure for analysis, each from POCh (Poland), and diethyl carbonate (98% purity), Fluka AG (Germany), were used for extraction. Each solvent was distilled prior to use. Pentadecane and heptadecane from Merck (Germany) were used as internal standards to determine the extraction percentage of phenols. Anhydrous sodium sulphate, pure for analysis, from POCh (Poland), was used as a drying agent. Tap water was used as a matrix in all of the experiments.

A volume of 2 ␮l stock solution containing phenols was added to 50 ml of a tap water sample. Next, 2 ml of an appropriate organic solvent with 4 ␮l of hydrocarbon standard mixture was added, shaken during 10 min and let to stand for phase separation. The hydrocarbon standard solutions in hexane contained 180 and 175 ng ␮l−1 of pentadecane and heptadecane, respectively. The resulting extract was dried over anhydrous sodium sulphate. The percent of extraction Ei was calculated according to Eq. (1): %Ei =

ms S i fi × 100 Ss mip

(1)

where m and S denote the mass of the considered compound in the analysed sample and the area of its corresponding peak, respectively. The subscripts i and s denote the considered compound and the standard. The subscript p denotes the initial mass of the considered compound in the analysed sample. The correction coefficient fi was calculated according to Eq. (2): fi =

mit Sst mst Sit

(2)

where the subscript t denotes the mass of the considered compound and the standard in the analysed test sample and the area of its corresponding peak, respectively. The test sample of phenols was prepared directly in hexane. The test sample of phenol acetyl derivatives was obtained in toluene by extraction from the aqueous sample. Each of the considered phenol derivative was totally extracted in three successive stages using the volume ratio of the aqueous solution to toluene equal to 5:1 [22].

2.2. Equipment and GC parameters

2.4. Derivatization

Gas chromatographic analyses were performed using a Carlo Erba HRGC 5300 Mega Series instrument, equipped with a split/splitless injector with septum-less head (JADE), a flame-ionisation detector (FID), a pressure regulation system and a PC-based data system Chromeleon (Gynkotek) for data acquisition. The analytical column was 30 m × 0.25 mm × 0.5 ␮m ZB-1 (Phenomenex). Helium (99.99% purity) was used as a carrier gas; inlet pressure was equal to 150 kPa. The chromatographic analyses were carried out in the following temperature programmed conditions: an initial oven temperature was held at 80 ◦ C for 3 min, then increased at 10 ◦ C/min to 260 ◦ C and, finally, held for 3 min. Detector temperature was 300 ◦ C. Injector temperature was 260 ◦ C. Analyses of phenols and phenol acetyl derivatives in hydrocarbons were carried out in the same conditions as the

Extraction depends upon several parameters, which characterize both the organic phase and the pollutants. The hydrophobicity of the separated compounds seems to be the most important parameter. Owing to this, in analytical applications phenols are usually converted to acetyl derivatives. Moreover, symmetrical peaks are obtained in gas chromatography for such derivatives. Acetic anhydride is often used, especially for derivatization of chloro- and alkylphenols. Acetylation can be carried out in situ in the water sample before the extraction step. Two microlitres of the stock solution containing phenols were added to 50 ml of a tap water sample and 0.5 g K2 CO3 was added to achieve a pH near 11. Then, 250 ␮l of acetic anhydride as a derivatizing reagent was added, and the solution was shaken for 15 min. Next, the extraction took place.

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The efficiencies of phenol removal by extraction with diethyl carbonate are given in Tables 1 and 2. Hexane and toluene were used as classical solvents to compare the effectiveness of phenol removal. The tables provide data on the percentages of extractions defined by Eq. (1), relative standard deviation (R.S.D.) and logarithm of distribution coefficient D defined by Eq. (3) relating the percentage of extraction with aqueous and organic volume phase ratios: Di =

Fig. 1. Chromatogram obtained for phenols extracted from water with diethyl carbonate: (1) phenol, (2) 4-methylphenol, (3) 2,4-dichlorophenol, (4) 2,3-dimethylphenol, (5) 4-chlorophenol, (6) 3-chlorophenol, (7) 4methoxyphenol, (8) 4-chloro-2-methylphenol, (9) 2,4,6-trichlorophenol, (10) 4-nitrophenol, (11) n-pentadecane, (12) 2,3,4,6-tetrachlorophenol and (13) n-heptadecane.

3. Results and discussion Typical chromatograms (Figs. 1 and 2) show that diethyl carbonate elutes at the beginning as a broad peak, which, however, does not cover the peaks of phenols or their acetyl derivatives. The peaks of phenols are sharp, symmetrical and well separated. There are no major differences in resolution of peaks when phenols are extracted directly or as acetyl derivatives. No interference from impurities present in water is observed. Also hexane and toluene are not observed to interfere with the peaks of phenols. As a result, the series of 11 phenols or their acetyl derivatives can be simultaneously extracted.

Fig. 2. Chromatogram obtained for phenol acetyl derivatives extracted from water with diethyl carbonate: (1) phenol, (2) 4-methylphenol, (3) 3-chlorophenol, (4) 4-chlorophenol, (5) 2,3-dimethylphenol, (6) 4methoxyphenol, (7) 4-chloro-2-methylphenol, (8) 2,4-dichlorophenol, (9) 2,4,6-trichlorophenol, (10) 4-nitrophenol, (11) n-pentadecane, (12) 2,3,4,6tetrachlorophenol and (13) n-heptadecane.

(Vw /Vo )%Ei 100 − %Ei

(3)

Extraction can depend upon the organic phase and the composition of the aqueous phase, including the acidity, ion strength or water activity and the concentration of solutes. The effect of pH can be connected with the formation of phenolates when phenols are extracted or with the hydrolysis of phenyl acetates. As a result, the convenient range of pH is limited to the range of 3–6, i.e., below pKa , for phenols (Table 2), and 6–7 for phenyl acetates to avoid any hydrolysis. Hexane is not a useful solvent for extraction of phenols without their derivatization. The extraction does not occur or is not effective enough because the phenols have too high affinity for water. The use of toluene gives better results. Only 4-nitrophenol is not extracted and extraction of 4-methoxyphenol is negligible (Table 3 ). The percentage of extraction changes from 0 to 70%, and log D increases up to 1.8. Diethyl carbonate is the most effective solvent. The extraction of 4-nitrophenol is still negligible (below 10%). However, the extraction of other phenols is in the range 40–100%. log D can achieve values as high as above 4. This high improvement of extraction with diethyl carbonate can be explained by the formation of a hydrogen bond between the hydroxyl group of phenols and the oxygen atom of the carbonate group: PhOH + O C(OEt)2 → PhOH · · · O C(OEt)2

(4)

Thus, diethyl carbonate is not a neutral solvent but it can be considered as a solvating reagent. The extraction of phenols can be further improved by addition of an electrolyte (Table 3), which are always present in actual technological solutions. The effect is the strongest for the more hydrophilic phenols. The effect of the presence of an electrolyte is weaker for the extraction of phenol acetyl derivatives. A decrease in the pH from 6 to 3 has only a negligible effect for the extraction of most phenols. However, an important improvement of extraction of 4-nitrophenol, 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol (compare Tables 2 and 3) is observed when pH is decreased from 6 to 3. Phenyl acetates are better extracted than phenols both with hexane and toluene. Again the poorest extraction is observed for 4-nitrophenol. The percentage of extraction changes up to 92% and log D does up to 3.7. However, the best extraction is observed again for diethyl carbonate. log D assumes very high values, as high as above 4, and several phenyl acetates are extracted quantitatively (in 100%). In this case, high extraction can be explained by high polarity of diethyl carbonate and the affinity of its structure to the structure of phenyl acetates.

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Table 1 Effect of solvent upon extraction percentage and log D of phenol acetyl derivatives, at aqueous and organic phase ratio equal to 25:1 and pH equal to 6 Compound

Hexane

Toluene

(%)a

(%)a

E Phenyl acetate 4-Methylphenyl acetate 3-Chlorophenyl acetate 4-Chlorophenyl acetate 2,3-Dimethylphenyl acetate 4-Methoxyphenyl acetate 4-Chloro-3-methylphenyl acetate 2,4-Dichlorophenyl acetate 2,4,6-Trichlorophenyl acetate 4-Nitrophenyl acetate 2,3,4,6-Tetrachlorophenyl acetate a

39 70 80 78 81 29 91 92 98 13 80

R.S.D. (%)

log D

E

1.5 2.2 1.0 2.9 1.6 7.9 2.7 1.1 1.7 24.0 4.2

1.204 1.766 2.000 1.948 2.028 1.009 2.403 2.459 3.090 0.572 2.000

79 89 92 90 90 79 95 94 95 85 78

Diethyl carbonate R.S.D. (%)

log D

E (%)a

R.S.D. (%)

log D

1.0 0.2 2.1 1.4 0.7 3.1 0.4 1.8 1.1 1.1 0.2

1.980 2.313 2.465 2.359 2.359 1.980 2.683 2.599 2.683 2.158 1.954

84 98 100 99 97 85 102 101 101 98 84

0.7 2.3 2.1 1.9 4.9 3.1 2.6 1.6 1.7 1.8 0.2

2.382 3.350 >4.0 3.660 3.170 2.415 >4.0 >4.0 >4.0 3.350 2.382

Average from three independent extractions, R.S.D.: relative standard deviation.

Thus, only diethyl carbonate enables efficient extraction both of underivatized phenols and their acetyl derivatives. Diethyl carbonate is a suitable solvent for the preconcentration step in phenol determination. However, more hydrophobic dialkyl carbonates having three or four carbon atoms in the alkyl group would be more convenient in technological applications. The Abraham model seems to be useful for the correlation of distribution coefficient log D with parameters, which characterize the structure and properties of the recovered pollutants [16–18]: log SP = c + a




αH 2 +b




β2H + sπ2H + rR2 + v

Vx 100

(5)

where SP refers to the property of interest for a series of solutes in a single solvent. The five parameters account for solute hydrogen bond acidity ( αH 2 ) and solute hydrogen  bond basicity ( β2H ), solute dipolarity (π2H ), solute excess molar refraction (R2 ) and solute molar volume (Vx /100). The values of Vx /100 can be calculated from the solute structure [19]. The values of the other four parameters have been determined for several hundred solutes, including various phenols, and can be found in literature [16–18].

The Abraham parameters for the considered pollutants are given in Table 4. The parameter αH 2 was neglected for phenyl acetyl derivatives, which had no free mobile  hydrogen able to form hydrogen bonds. The parameters ( β2H , π2H , R2 and Vx /100) for acetyl derivatives were calculated from the following relationship [20]: Pi (solute acetate) = Pi (phenyl acetate) + [Pi (solute) − Pi (phenol)]

(6)

Molar volumes were calculated from increments given by McGowan and Sowada [19]. Additionally, octanol–water partition coefficient of the separated pollutants, calculated using the Hansch and Leo method [21], was considered as an alternative measure of the pollutant hydrophobicity. Octanol–water partition coefficient (Kow ) is often used instead of molecular volume because in our opinion the solute hydrophobicity is better characterized by log Kow than molecular volume and satisfactory results were obtained [9,13,22]. Unfortunately, in this particular case of extraction using diethyl carbonate weak correlation without the parameter log Kow was obtained (Eq. (10)). It was found that both the percentage of extraction and the logarithm of the distribution coefficient could be satisfactorily correlated with the

Table 2 Extraction percentage and log D of phenols, at aqueous and organic phase ratio equal to 25:1 and pH equal to 3 Compound

pKa

Hexane

Toluene

(%)a

(%)a

E Phenol 4-Methylphenol 2,4-Dichlorophenol 2,3-Dimethylphenol 4-Chlorophenol 3-Chlorophenol 4-Methoxyphenol 4-Chloro-3-methylphenol 2,4,6-Trichlorophenol 4-Nitrophenol 2,3,4,6-Tetrachlorophenol

9.92 10.26 7.87 10.5 9.41 9.12 9.48 9.55 7.42 7.16 5.22

NE NE 34 8 NE NE NE 5 72 NE 79

R.S.D. (%)

log D

E

– – 6.8 6.6 – – – 9.6 4.7 – 3.6

– – 1.110 0.337 – – – 0.119 1.808 – 1.973

12 15 75 51 24 26 3 58 95 NE 80

R.S.D.: relative standard deviation, NE: not extracted, –: not calculated. a Average from three independent extractions.

Diethyl carbonate R.S.D. (%)

log D

E (%)a

R.S.D. (%)

log D

5.6 9.9 1.3 0.1 2.4 6.4 20.3 1.0 0.5 – 5.2

0.539 0.651 1.882 1.422 0.904 0.950 −0.105 1.545 2.683 – 2.007

42 61 90 81 87 82 29 92 98 33 85

6.5 4.1 11.1 3.9 3.5 7.2 2.1 5.3 9.5 6.5 6.5

1.557 1.891 2.651 2.327 2.522 2.355 1.308 2.758 3.390 1.389 2.450

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Table 3 Effect of salting-out upon extraction percentage and log D of phenols, at aqueous and organic phase ratio equal to 25:1 and pH equal to 6 Compound

Electrolyte content (g NaCl/100 g water)

Hexane

Phenol

0 3 10

NE NE NE

4-Methylphenol

0 3 10

NE NE NE

2,4-Dichlorophenol

0 3 10

24 26 36

2,3-Dimethylphenol

0 3 10

9 9 15

4-Chlorophenol

0 3 10

NE NE NE

3-Chlorophenol

0 3 10

4-Methoxyphenol

0 3 10

4-Chloro-3-methylphenol

0 3 10

5 4 9

2,4,6-Trichlorophenol

0 3 10

5 8 13

4-Nitrophenol

0 3 10

NE NE NE

2,3,4,6-Tetrachlorophenol

0 3 10

NE NE NE

a

E

(%)a

Toluene R.S.D. (%)

Diethyl carbonate

log D

E (%)

R.S.D. (%)

log D

E (%)

R.S.D. (%)

log D

– – –

– – –

11 12 11

8.9 11.0 8.3

0.497 0.538 0.493

40 50 66

3.0 1.7 4.3

1.523 1.620 1.811

– – –

– – –

15 18 25

0.0 7.6 10.8

0.651 0.745 0.924

60 72 82

3.6 4.3 6.1

1.873 2.030 2.181

14.4 12.3 1.0

0.897 0.944 1.148

59 70 70

6.7 5.5 9.8

1.56 1.77 1.77

88 98 100

1.9 2.7 2.6

2.561 3.310 >3.8

21.3 16.6 2.0

0.393 0.393 0.645

47 55 61

7.1 7.7 11.1

1.35 1.49 1.60

78 88 91

2.1 4.9 1.0

2.246 2.485 2.528

– – –

– – –

21 24 34

11.3 10.2 9.5

0.829 0.903 1.113

84 96 99

1.5 1.6 4.7

2.419 3.0 3.5

NE NE NE

– – –

– – –

21 24 36

5.6 7.6 7.3

0.829 0.903 1.151

83 92 95

3.4 5.9 4.6

2.388 2.680 2.802

NE NE NE

– – –

– – –

2 3 7

1.3 20.0 6.5

−0.286 −0.106 0.278

29 41 49

8.8 2.5 6.2

1.310 1.462 1.506

11.2 13.1 4.1

0.119 0.018 0.393

53 61 67

11.3 12.0 7.3

1.457 1.598 1.709

92 101 100

2.5 2.6 4.1

2.760 >3.9 >3.8

26.9 11.7 10.9

0.119 0.337 0.572

35 49 64

11.8 11.3 11.6

1.136 1.386 1.651

56 78 96

3.5 5.6 5.0

1.804 2.169 2.903

– – –

– – –

0 1 1

67.7 101.8 16.7

– −0.592 −0.594

3 12 17

26.5 49.4 1.2

0.189 0.754 0.834

– – –

– – –

23 36 52

5.3 14.2 9.2

0.880 1.154 1.436

38 62 78

2.4 7.1 2.2

1.486 1.832 2.073

Average from three independent extractions, R.S.D.: relative standard deviation, NE: not extracted.

pollutant’s parameters. Both these relationships have similar statistical characteristics. However, those obtained for the distribution coefficient only are presented in this work because the distribution coefficient is of importance in the physical–chemical aspect while the percentage of extraction depends on the concentration of pollutants in the aqueous feed. The selected equations for the considered systems together with their statistical assessment are as follows: • hexane
log D = −(2.354 ± 0.316) αH 2 − (2.494 ± 0.943)
× β2H − (0.556 ± 0.261)π2H + (2.992 ± 0.385) R2 = 0.941,

Vx , 100

n = 20,

S.D. = 0.374,

F =6

• toluene


log D = (0.472 ± 0.434) − (1.71 ± 0.24) αH 2
− (1.827 ± 0.418) β2H − (0.128 ± 0.021)R2 + (2.298 ± 0.306) R2 = 0.972,

Vx , 100

n = 20,

S.D. = 0.144,

F = 158

(8)

• diethyl carbonate


log D = (3.258 ± 0.659) − (2.93 ± 0.32) αH 2
− (4.930 ± 0.566) β2H + (2.091 ± 0.489) Vx , n = 20, R2 = 0.937, 100 S.D. = 0.224, F = 95

× (7)

(9)

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Table 4 Recovered pollutants and their Abraham parameters and octanol/water partition coefficients  H  H No. Pollutant α2 β2

π2H

R2

Vx /100

log Po/w

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

0.89 0.87 0.84 0.81 1.08 1.06 1.17 1.02 0.53 1.72 1.13 1.01 1.30 1.32 1.05 1.41 1.26 1.08 0.77 1.96 NF NF

0.805 0.820 0.960 0.850 0.915 0.909 0.900 0.920 0.986 1.070 0.661 0.660 0.765 0.771 0.706 0.756 0.776 0.816 0.842 0.926 NF NF

0.775 0.916 1.020 1.057 0.898 0.898 0.975 1.038 1.142 0.949 1.073 1.214 1.195 1.195 1.354 1.272 1.336 1.317 1.440 1.247 1.265 1.562

1.47 1.97 3.06 2.48 2.39 2.50 1.58 3.10 3.69 1.91 1.49 2.11 2.23 2.23 2.68 1.54 2.78 2.88 3.52 1.50 4.45 4.17

a

Phenol 4-Methylphenol 2,4-Dichlorophenol 2,3-Dimethylphenol 4-Chlorophenol 3-Chlorophenol 4-Methoxyphenol 4-Chloro-3-methylphenol 2,4,6-Trichlorophenol 4-Nitrophenol Phenyl acetate 4-Methylphenyl acetate 3-Chlorophenyl acetate 4-Chlorophenyl acetate 2,3-Dimethylphenyl acetate 4-Methoxyphenyl acetate 4-Chloro-3-methylphenyl acetate 2,4-Dichlorophenyl acetate 2,4,6-Trichlorophenyl acetate 4-Nitrophenyl acetate 2,3,4,6-Tetrachlorophenol 2,3,4,6-Tetrachlorophenyl acetate

0.60 0.57 0.53 0.53 0.67 0.69 0.57 0.65 0.84 0.82 0 0 0 0 0 0 0 0 0 0 NFa 0

0.31 0.32 0.19 0.36 0.21 0.15 0.48 0.23 0.22 0.26 0.54 0.56 0.38 0.43 0.59 0.71 0.46 0.42 0.45 0.49 NF NF

Not found.

• diethyl carbonate




log D = (5.890 ± 0.426) − (3.83 ± 0.37) αH 2
−(4.949 ± 0.823) β2H , n = 20, R2 = 0.867,

S.D. = 0.326,

F = 63

(10)

where n, R2 , S.D. and F denote the number of compounds, determination coefficient, standard deviation and Fisher–Snedecor function, respectively. Fig. 3 demonstrates agreement between the observed and the calculated values of the distribution coefficient (from Eqs. (7)–(9)). Only for 4-nitrophenol significant deviation

from the correlation line for diethyl carbonate is observed. This can be explained most probably by the formation of hydrogen bonds between the nitro groups of phenol and water molecules. Each equation contains the strongly positive term Vx /100 indicating the importance of the pollutant’s hydrophobicity. Extraction increases with an increase in the pollutant’s hydrophobicity, as could be expected from the Traube rule. However, the effect of other parameters is not well identified.

4. Conclusions Diethyl carbonate is an effective solvent that enables high recoveries both of phenols and their acetyl derivatives. The high recovery of phenols can be explained by the formation of a hydrogen bond between the hydroxyl group of phenols and the oxygen atom of the carbonate group. The efficiency of extraction depends mainly upon phenol hydrophobicity and significantly increases with an increase in the solute molar volume. Using the Abraham model, distribution coefficients are well correlated with the pollutant parameters. The hydrophobicity of the pollutants, as measured by molar volume, is the most important factor affecting extraction of phenols and their acetyl derivatives while the effect of other parameters is not well identified.

Acknowledgement Fig. 3. Comparison of experimental and calculated values of log D for phenols and phenol acetyl derivatives obtained for extraction with diethyl carbonate (䊉), toluene (
) and hexane (+).

The work was supported by the Polish State Committee for Scientific Research—KBN 1318/T09/2004/26.

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