Article pubs.acs.org/jced
Extraction of 8‑Hydroxyquinoline by Tributylphosphate in Kerosene De-Liang Li,* Yuan-Yuan Guo, Zhi-Xian Chang, and Xiao-Li Feng Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China ABSTRACT: Extractions of 8-hydroxyquinoline (8-HQ) by tributylphosphate (TBP) in kerosene, as well as pure TBP and kerosene, were carried out. The investigations involved the equilibrium pH (pHeq) of 3.0 to 11.5, initial 8-HQ concentrations of (0.5 to 5.0) mmol·L−1, and initial TBP concentrations of (0.05 to 1.0) mol·L−1. The distribution ratio (D) was found to be highly dependent on pHeq, and larger D values appeared at pHeq range of 6.3 to 8.4. The distribution constants (KD) for pure TBP and kerosene were calculated to be 236.6 and 26.98, respectively. The experimental KD for TBP/kerosene was expressed by KD,TBP, KD,kerosene, and the volume fractions of TBP or kerosene. The mix of TBP and kerosene was found to be a nonideal process. The deviation of KD from the ideal is proportional to the product of volume fractions of TBP and kerosene. The (1:1) TBP:8-HQ complex was verified by fitting experimental data, and the apparent extraction reaction equilibrium constant was calculated to be 64.58 L·mol−1. The results are of significant importance for the practical separation of 8-HQ from diluents.
1. INTRODUCTION 8-Hydroxyquinoline (8-HQ) is an important raw material and intermediate used in chemical, pharmaceutical, and pesticide industries. Its sulfate and copper salts are excellent preservatives, disinfectants, and mold inhibitors. It is also known as a chelating extractant and complexometric indicator for determining metal ions such as La(III), Pr(III), Nd(III), and Cu(II) in analytical chemistry. Because of toxicity, carcinogenicity, and bioaccumulation even at a low level of concentration, 8-HQ is always reported as a hazardous compound. For example, Wu et al.1 reported that 8-HQ could inhibit the development of paramisgurnus dabryanus embryo and larva with obvious teratogenic and lethal effects, and the fish mortalities increased with the increase of 8-HQ concentration. However, a large amount of wastewaters containing 8-HQ are being drained during our present chemical production process. It has caused serious potential pollution. What’s worse, the studies on treatment of this kind wastewater are rarely reported until now. Based on adsorption technology, Erdem et al.2 and Opera et al.3 treated 8-HQ solutions using neutral bentonite and cerusite, which supported important information for separation and recovery of 8-HQ from effluent. Advanced oxidation processes, like ozonation technology and advanced electrocatalysis oxidation, are of high efficiencies for removing 8-HQ, but the processes are often accompanied with high costs. Though biodegradation was considered as a clean technique for wastewater treatment, the selection and culture of characteristic micro-organisms still required in-depth studies. Extraction, characterized by its easiness to operate and convenience to scale up, demonstrates a wide application prospect in purification and separation including wastewater treatment and has received great attention for several decades. © 2012 American Chemical Society
Research on extraction of 8-HQ using more than 20 organic compounds has been reported by previous authors and the distribution constants (KD) are collected and listed by Robak et al. in ref 4. It shows that KD values for the solvents of chloralkanes, aromatics, ketones, and alcohols are commonly higher than 100, while the values for alkanes are lower than 30. In spite of these results, there is no practical extraction process for the separation of 8-HQ, because these solvents are either of toxicity (i.e., chloroform and toluene) or of high solubility in water (i.e., 1-octanol and pentanone), while the low-toxic alkanes are shown with low KD values. Reactive extraction, which is based on the reversible reaction of the specific functional groups of solute and extractant, is an effective separation technique for organic compounds. It is receiving increasing attention for separation polar organics recently.5−13 Especially in China, it has been successfully used for cleaning processes of wastewaters containing phenols or amines. As far as our literature collection, few works have been reported on the separation of 8-HQ by this technique. Hence, in the present work, extraction of 8-HQ using tributylphosphate (TBP) in kerosene was studied in detail. The factors that affect extraction equilibrium, such as equilibrium aqueous pH (pHeq) and initial 8-HQ and TBP concentrations, were investigated. An expression of D for the practical extraction process was proposed. The complexation ratio and the apparent extraction reaction equilibrium constant were also calculated. Received: June 27, 2012 Accepted: August 30, 2012 Published: September 10, 2012 2817
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2. EXPERIMENTAL SECTION 2.1. Chemicals. 8-Hydroxyquinoline (8-HQ, purity of ≥ 99.5 %, Sinopharm Chemical Reagent Co., Ltd.) and tributylphosphate (TBP, volume fraction of ≥ 99 %, density of 0.976 g·cm−3, Luoyang Aoda Chemical Reagent Co., Ltd. Henan, China) were used as received. The structures of 8-HQ and TBP are shown in Figure 1. Kerosene was purchased from a local petrochemical plant and was pretreated as in ref 13. High-purity water was used throughout the experimental procedure.
calibration curve equation was calculated as A = 42.88c + 0.01483 (correlation coefficient, 0.9996; linear scope, (0.001 to 0.04) mmol·L−1), where A is the absorbance and c represents the concentration of 8-HQ in aqueous solution. The loading of 8-HQ in organic phase was calculated by mass balance. The deviation of this method did not exceed 3 %.
3. RESULTS AND DISCUSSION 8-HQ has a Lewis acid group −OH and a Lewis base group −N−, exhibiting Lewis acid−base character. There are three forms in aqueous solution, namely, the charged cations (H2Q+), the neutral molecule (HQ), and the anions (Q−). Two dissociation balances exist in aqueous solution as follows: K a1
H 2Q+ ← → HQ + H+ K a2
HQ ← → Q− + H+
K a1 =
K a2 =
[HQ][H+] [H 2Q+]
[H+][Q−] [HQ]
(1)
(2)
14
with pKa1 and pKa2 are 4.91 and 9.81, respectively. pH plays a vital role on the existing forms of 8-HQ in aqueous solution. At lower pH (pH < pKa1), H2Q+ dominates, while HQ dominates at intermediate pH (pKa1 < pH < pKa2) and Q− appears at higher pH (pH > pKa2). The molar fractions of different species of 8-HQ under variable pH are shown in Figure 3. The extraction behaviors will be different at different pH.
Figure 1. Structures of 8-HQ (A) and TBP (B).
2.2. Procedures. TBP was diluted by kerosene with the initial concentration range of (0.05 to 1.0) mol·L−1. 8-HQ solutions were prepared by dissolving solid 8-HQ in 0.005 mol·L−1 sulfuric acid with the initial concentration range of (0.5 to 5) mmol·L−1. All extraction experiments were performed by shaking equal volumes (15 mL) of initial aqueous and organic phases, placed in 100 mL flasks, for 3 h in a SHZ-B constant temperature water bath-vibrator (Shanghai Yuejin Medical Instruments Factory) at 180 ± 10 rpm, followed by settling of the mixture for 4 h at a fixed temperature (298 ± 0.5 K) and pressure (101.3 kPa). The operation conditions were preliminarily tested to be sufficient for equilibrium. After separation, aqueous pHeq was determined by a HI1200B composite electrode with a pH meter (HANNA pH 211 Microprocessor pH meter, Italy) with a deviation of ± 0.01. The different pHeq values were obtained by adjusting the initial aqueous pH with a little saturated NaOH or diluent H2SO4 solution. The final aqueous solution was adjusted to pH of 2.0 and analyzed at 251 nm (maximum adsorption wavelength of 8-HQ under selected conditions) on an UV spectrometer (LabTech UV-2000, Beijing Labtech, Ltd.). The typical absorption spectrum and calibration curves are shown in Figure 2. The
Figure 3. Molar fractions of different species for 8-HQ.
Table 1 and Figure 4 demonstrate D values for pure kerosene and (0.1, 0.3, and 0.7) mol·L−1 TBP in kerosene with equilibrium pH (pHeq) in aqueous solution as variable. It is obvious that D values for a certain organic composition increase first and then decrease with pH increasing from 2 to 11, and the peak values of D always appear at the pHeq range of 6.2 to 8.3. This change trend of D values was suddenly found to be similar to that of the molar fraction of neutral 8-HQ in aqueous solutions. It suggests the transfer of 8-HQ is highly dependent on the concentration of neutral 8-HQ in aqueous phase; in other words, TBP might mainly interact with neutral 8-HQ during the extraction process. Based on the results above, all of the following equilibrium data were collected under pHeq of 6.2 to 8.3, in which 8-HQ dominated as a neutral form. The extraction of 8-HQ using TBP/kerosene mixture involves two parts, namely, the extraction of 8-HQ by kerosene and the extraction of 8-HQ
Figure 2. Adsorption spectra of 8-HQ. The insert is the calibration curve. 2818
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Table 1. Data for the Extraction of 5 mmol·L−1 8-HQ Using TBP in Kerosene ctot,aq
init. conc. TBP mol·L 0 0 0 0 0 0 0 0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.7 0.7 0.7 0.7 0.7 0.7
−1
equil. pH in aq. phase 3.14 3.80 5.96 7.36 7.88 8.78 10.15 10.86 3.67 4.60 6.35 7.02 8.03 9.26 9.66 10.57 3.83 4.82 7.02 7.96 9.56 10.67 11.26 3.73 5.12 7.60 9.56 10.73 11.31
The physical extraction of 8-HQ by kerosene can be written as follows,
ctot,org −1
mmol·L
3.809 2.71 0.333 0.206 0.217 0.276 0.744 2.44 2.08 0.469 0.183 0.159 0.162 0.278 0.356 1.26 1.40 0.327 0.123 0.117 0.263 1.21 2.18 1.19 0.150 0.0791 0.172 0.863 1.82
KD,kerosene
−1
mmol·L 1.19 2.29 4.67 4.79 4.78 4.72 4.26 2.56 2.92 4.53 4.82 4.84 4.84 4.72 4.64 3.74 3.61 4.67 4.88 4.88 4.74 3.79 2.82 3.81 4.85 4.92 4.83 4.14 3.18
KD,kerosene =
(HQ)aq ←⎯⎯⎯⎯⎯⎯→ (HQ)org
D 0.313 0.842 14.0 23.3 22.0 17.1 5.72 1.05 1.40 9.67 26.4 30.5 29.9 17.0 13.0 2.96 2.57 14.3 39.5 41.7 18.0 3.14 1.29 3.20 32.3 62.2 28.0 4.79 1.75
[HQ]org [HQ]aq (3)
where [HQ] represents concentration of HQ (mol·L−1) and subscripts “aq” and “org” refer to the species in aqueous or organic phase, respectively. KD,kerosene is distribution constant of 8-HQ between water and kerosene. The chemical extraction of 8-HQ using TBP by the formation of a Lewis acid−base complex can be represented as, K
(HQ)aq + n(TBP)org ↔ (HQ·nTBP)org K=
[(HQ·nTBP)org ] [(HQ)aq ]· [(TBP)org ]n
(4)
where K is the apparent extraction reaction equilibrium constant and n is the molecule number of TBP per complex. The distribution ratio, D, represents the ratio of the total concentrations (ctot) of 8-HQ in both phases and can be defined as, ctot,org D= ctot,aq (5) Because all D values were received under different pHeq and pHeq have a great effect on D (as discussed above), the direct comparison of D for each system seems unreasonable. Thus, the distribution constant, KD, which is expressed as the ratio of total concentration of 8-HQ in the organic phase to the concentration of neutral 8-HQ in aqueous phase, here is proposed. It could be calculated using eq 6. ctot,org KD = [(HQ)aq ] (6) The total concentration of 8-HQ in aqueous solution is equal to the sum of equilibrium concentration of its neutral and ionic species; namely, ctot,aq = [(H 2Q+)aq ] + [(HQ)aq ] + [(Q−)aq ]
(7)
From eqs 1, 2, and 7, it follows that, ctot,aq
⎛ [H+]aq K a2 ⎞ ⎟ = [(HQ)aq ]⎜⎜ +1+ [H+]aq ⎟⎠ ⎝ K a1
(8)
Then from eqs 5, 6, and 8, D is received as eq 9. D=
−1
Figure 4. D of 8-HQ (initial concentration: 5 mmol·L ) vs pHeq. Initial concentration of TBP (in mol·L−1): ■, 0; ●, 0.1; ▲, 0.3; and ▼, 0.7. Lines were plotted using the proposed model as described in the paper.
KD [H+]aq K a1
+1+
K a2 [H+]aq
=
KD 10
pK a1− pHeq
+ 1 + 10 pHeq − pKa2 (9)
which permits us to evaluate D of 8-HQ in the whole range of pHeq. For a particular composition of TBP/kerosene mixture, KD can be calculated by linearly fitting ctot,org vs [HQ]aq. Systematical experimental studies were carried out by extracting 8-HQ solutions with different initial concentrations using TBP in kerosene at different levels of concentration. The experimental data are listed in Table 2. Figure 5 shows the plots of ctot,org vs [HQ]aq. It was observed that, for a certain TBP/ kerosene, ctot,org seemed in a linear relationship with [HQ]aq; thus ctot,org was linearly fitted with [HQ]aq. Because when the
by TBP. Kerosene is an inert and nonploar solvent without any active group, and it cannot react with 8-HQ through chemical bond; thus its extraction for 8-HQ could be consider as physical extraction. However, TBP has a phosphoryl group with a high polarity, which enables it to act as a strong Lewis base, and as a result, it could react with hydroxyl containing organic like 8HQ to form a Lewis acid−base complex, so its extraction for 8HQ could be thought as chemical extraction.11,15 2819
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Table 2. Experimental Results of Extraction Equilibria of 8-HQ by TBP in Kerosene init. conc. of TBP
init. conc. of 8-HQ
mol·L−1
mmol·L−1
pHeq
0 0 0 0 0 0 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.3 0.3 0.3 0.3 0.3 0.5 0.5 0.5 0.5 0.5 0.5 0.7 0.7 0.7 0.7 0.7 0.7 1 1 1 1 1 1 3.665 3.665 3.665 3.665 3.665 3.665
0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5 0.5 1 2 3 4 5
7.76 7.83 7.69 7.86 8.07 7.95 6.96 7.11 6.98 7.26 7.66 7.92 7.48 7.38 7.52 7.25 7.67 7.92 7.28 6.98 6.97 7.36 7.6 7.96 7.25 7.19 6.9 7.29 7.67 7.76 7.2 7.14 6.95 6.92 7.63 7.87 7.12 7.08 6.88 7.16 7.45 7.25 5.92 6.09 6.19 6.63 6.58 7.27
ctot,aq
[HQ]aq
ctot,org
mmol·L−1 mmol·L−1 mmol·L−1 0.0198 0.0415 0.0799 0.110 0.146 0.175 0.0249 0.0427 0.0784 0.101 0.135 0.169 0.0187 0.0340 0.0650 0.0873 0.118 0.152 0.0151 0.0269 0.0518 0.0659 0.0898 0.117 0.0124 0.0240 0.0402 0.0521 0.0740 0.0961 0.0102 0.0199 0.0332 0.0457 0.0616 0.0788 0.00817 0.0160 0.0261 0.0350 0.0489 0.0637 0.00311 0.00589 0.00952 0.0131 0.0168 0.0206
0.0196 0.0410 0.0791 0.108 0.143 0.173 0.0246 0.0424 0.0776 0.0998 0.134 0.166 0.0185 0.0337 0.0645 0.0866 0.117 0.150 0.0150 0.0266 0.0512 0.0654 0.0890 0.115 0.0123 0.0239 0.0397 0.0517 0.0733 0.0951 0.0101 0.0197 0.0329 0.0452 0.0610 0.0779 0.0081 0.0159 0.0258 0.0348 0.0486 0.0632 0.00283 0.00552 0.00904 0.0129 0.0165 0.0204
0.480 0.959 1.92 2.89 3.85 4.82 0.475 0.957 1.92 2.90 3.87 4.83 0.481 0.966 1.94 2.91 3.88 4.85 0.485 0.973 1.95 2.93 3.91 4.88 0.488 0.976 1.96 2.95 3.93 4.90 0.490 0.980 1.97 2.95 3.94 4.92 0.492 0.984 1.97 2.96 3.95 4.94 0.497 0.994 1.99 2.99 3.98 4.98
D 24.2 23.1 24.0 26.4 26.5 27.5 19.1 22.4 24.5 28.8 28.7 30.0 25.8 28.4 29.8 33.4 32.8 31.9 32.1 36.2 37.6 44.5 43.6 41.7 39.2 40.6 48.8 56.6 53.1 51.1 48.1 49.3 59.2 64.6 64.0 62.4 60.2 61.6 75.5 84.6 80.7 77.5 160.0 168.8 209.2 227.5 236.4 242.1
Figure 5. ctot,org vs [HQ]aq. Initial concentration of TBP (in mol·L−1): ■, 0; ●, 0.05; ▲, 0.1; ▼, 0.3; ◀, 0.5; ▶, 0.7; ◆, 1.0; and ★, 3.665.
For an industrial process, it is much more convenient to predict KD values using the composition fractions of organic phase. So, the relationships of KD values with the volume fractions of TBP (φTBP) or kerosene (φkerosene) were fitted respectively (quadratic curves in Figures 6 and 7). KD values are calculated to follow the eqs 10 and 11 with both R2 close to 1. 2 KD = 22.85φTBP + 187.01φTBP + 26.73
KD =
2 22.85φkerosene
− 232.72φkerosene + 236.6
(10) (11)
Table 3 demonstrates that KD values for pure kerosene and pure TBP are 26.98 and 236.6, respectively. Assuming the mixing process of TBP and kerosene is ideal, for TBP/kerosene mixtures, K D values should be equal to the sum of corresponding part of KD supplied by TBP and kerosene; that is, KD,ideal = KD,TBPφTBP + KD,keroseneφkerosene (12) KD,ideal values for each tested organic phase are also calculated in Table 3 and plotted in Figures 6 and 7 (dashed lines). They are observed with some deviation (Δ, eq 13) from KD values (experimental ones), which indicates a nonideal mixing process of TBP and kerosene. Δ = KD,ideal − KD
(13)
From eqs 10 to 13, it follows that, 2 Δ1 = 22.59φTBP − 22.85φTBP + 0.25
≈ 22.85φTBP(1 − φTBP)
(14)
2 Δ2 = 23.12φkerosene − 22.85φkerosene
≈ 22.85φkerosene(1 − φkerosene)
(15)
Integrate eqs 14 and 15 into eq 16, Δ = 22.85φ(1 − φ)
(16)
where φ represents the volume fraction of TBP or kerosene. Δ here represents the negative effect of mixing process of TBP and kerosene on KD. It is known that kerosene is an inert and nonpolar solvent, and its action on other organic molecule is mainly by dispersion forces, while TBP is a typical polar solvent containing functional group PO, which makes it interact with other molecules through hydrogen bonding action, and so forth. As to the present solute (8-HQ), it is a polar organic compound containing functional group −OH, and it might act
initial concentration of 8-HQ is 0 mol·L−1, both ctot,org and [HQ]aq are also equal to 0 mol·L−1, the fitted lines are fixed through the (0,0) point. According to eq 6, the lines' slopes correspond to the KD values for TBP/kerosene. The linear equations for pure kerosene, TBP/kerosene, and pure TBP with correlation coefficients (R2) are listed in Table 3. All R2 values are received as higher than 0.997, indicating a good fitted result of all experimental data. 2820
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Table 3. Volume Fractions of TBP and Kerosene and KD Values for Each System init. conc. TBP
a
mol·L−1
φTBP
φkerosene
fitted eqsa
0 0.05 0.1 0.3 0.5 0.7 1 3.665
0 0.0136 0.0272 0.0815 0.1359 0.1902 0.2718 1
1 0.9864 0.9728 0.9185 0.8641 0.8098 0.7282 0
y y y y y y y y
= = = = = = = =
26.98x 28.33x 32.41x 42.55x 52.28x 63.12x 79.26x 236.6x
R2
KD ± sb
KD,ideal
0.9951 0.9977 0.9984 0.9968 0.9958 0.9973 0.9964 0.9962
26.98 ± 0.53 28.33 ± 0.81 32.41 ± 0.54 42.55 ± 0.98 52.28 ± 1.38 63.12 ± 1.34 79.26 ± 1.95 236.6 ± 6.11
26.98 29.83 32.68 44.06 55.47 66.85 83.95 236.6
x refers to [HQ]aq, and y represents ctot,org. bs is the uncertainty of calculated KD.
KD = =
ctot,org [HQ]aq [HQ·nTBP]org + [HQ]org [HQ]aq
= K[TBP]norg + Kkeroseneφkerosene − Δ
(17)
Because of the nonideal mix of TBP and kerosene, Δ here is subtracted. Supposing that the initial concentration of TBP is B0, which is far higher than that of 8-HQ, the concentration is of no change before and after extraction. Thus, eq 17 is revised as, KD − (Kkeroseneφkerosene − Δ) = KB0n
(18)
If logarithm is adopted by eq 18, it is deduced that
Figure 6. KD (solid line) or KD,ideal (dash line) vs φTBP.
log[KD − (Kkeroseneφkerosene − Δ)] = log K + n log B0 (19)
Thus, the plot of log[KD − (KD,keroseneφkerosene − Δ)] vs log B0 would yield a straight line with the slope of n and the intercept of log K, from which K can be obtained. Figure 8
Figure 7. KD (solid line) or KD,ideal (dash line) vs φkerosene.
with kerosene mainly through dispersion forces and act with TBP mainly by hydrogen bonding. Because the hydrogen bonding action is far stronger than dispersion force action, we thought that the deviation (Δ) mainly derived from the effect of TBP on the part KD value of kerosene rather than the effect of kerosene on the part KD value of TBP; in other words, kerosene did not affect the extraction capacity of TBP vs 8-HQ, while TBP affected the extraction capacity of kerosene vs 8-HQ. The extraction of 8-HQ using TBP/kerosene mixture contains a chemical extraction part and physical part; in addition to taking the nonideal mixing process into account, KD for the mixture can also be expressed as eq 17.
Figure 8. log[KD − (KD,keroseneφkerosene − Δ)] vs log B0.
shows that the linear slope n is nearly 1 and log K is 1.8101 with the correlation coefficient R2 = 0.9912. This result suggested that a (1:1) TBP:8-HQ complex was formed in the extraction process with K = 64.58 L·mol−1. Finally, KD can be calculated from eqs 10, 11, 12, 13, 16, and 17, respectively; then D is received using eq 9, which allows us to predict the extraction efficiency of any TBP/kerosene + 8-HQ system.
4. CONCLUSION The extraction of 8-HQ by using TBP dissolved in kerosene was investigated. The distribution ratio was found to be highly 2821
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(13) Chang, Z. X.; Xu, M.; Zhang, L.; Li, D. L. Reactive extraction of o-, m-, and p-aminophenol using trialkylphosphine oxide/kerosene. J. Chem. Eng. Data 2012, 57, 2030−2036. (14) Dean, J. A. Lange’s handbook of chemistry; McGraw-Hill Book Company: New York, 1985. (15) Wasewar, K. L.; Shende, D. Z. Reactive extraction of caproic acid using tri-n-butyl phosphate in hexanol, octanol, and decanol. J. Chem. Eng. Data 2011, 288−297.
dependent on equilibrium pH in aqueous solution. The distribution constant for each TBP/kerosene were fitted and expressed by the volume fractions of TBP (or kerosene) in the mixture. It was also found that a (1:1) TBP:8-HQ complex was formed with the apparent extraction reaction equilibrium constant K = 64.58 L·mol−1. The present results permit us to use the proposed expressions for KD and D to predict the extraction efficiency of any TBP/kerosene mixture for 8-HQ, which would be helpful for an industrial process of extraction of 8-HQ using TBP/kerosene.
■
AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86 378 3881589. E-mail address: lideliang@ henu.edu.cn. Funding
This work is supported by a Major Project of Science and Technology, Education Department of Henan Province, China (No. 12A610001). Notes
The authors declare no competing financial interest.
■
REFERENCES
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