Reactive Extraction of p-Nitrophenol Using Tributylphosphate in

Jan 27, 2016 - In this work, the reactive extraction equilibrium of p-nitrophenol (PNP) using tributylphosphate (TBP) dissolved in solvent naphtha and...
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Reactive Extraction of p‑Nitrophenol Using Tributylphosphate in Solvent Naphtha or n‑Octanol Bing Cui, Jing-Chao Gong, Mu-Hua Duan, Zhi-Xian Chang, Ling-Ling Su, Wen-Jing Liu, and De-Liang Li* Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China ABSTRACT: In this work, the reactive extraction equilibrium of p-nitrophenol (PNP) using tributylphosphate (TBP) dissolved in solvent naphtha and noctanol has been studied, respectively. The effects of equilibrium pH (pHeq), initial TBP, and PNP concentrations on the distribution ratio (D) and the degree of extraction were discussed. The TBP/solvent naphtha showed higher D values than that of TBP/n-octanol. The larger D values were also observed at pHeq in the range of 2−5.5. pHeq had negligible influences on the structure of TBP/PNP complexes based on IR spectra analysis. TBP mainly reacted with the neutral form of PNP by forming 1:1 (TBP/PNP) complexes through a hydrogen bond between phosphoryl in TBP and hydroxyl in PNP. The model parameters were calculated by fitting the experimental data. D values calculated from the model were in good agreement with experimental ones. In addition, the back-extraction rate was found to be reached 99.9% using 50 mmol·kg−1 NaOH as stripping agent.

1. INTRODUCTION With the rapid development of industry, a growing number of toxic and hazardous pollutants accumulate in the biosphere. Phenolic compounds are important raw materials for fine chemicals and have been widely used in the fields of pharmaceuticals, feeds, resins, dyes, pesticides, and so forth.1 These phenolic compounds are highly toxic by ingestion or inhalation at the low concentrations.2 Nitrophenol and its isomers, which have been widely used in the production of pesticides, insecticides, and herbicides,3 have caused great harm to the environment and humans. Meanwhile, nitrophenol has been listed on the U.S. Environmental Protection Agency 129 kinds of priority pollutants.4 During the application process of these compounds, some of them will be released into environment and contaminate rivers and groundwater and thus cause damage to human health due to their high toxicity.5 Therefore, how to remove p-nitrophenol from water becomes an important issue currently. The treatment of p-nitrophenol wastewater is divided into two categories. One is based on chemical reactions. It emphasizes that the material has a reaction and destroy process,6−8 such as biological degradation,9,10 thermal decomposition,11,12 and advanced oxidation.13−16 The other is based on physical processes, such as adsorption,17−19 membrane separation,20 and liquid−liquid extraction.21−27 However, the practical application of the above-mentioned categories have many problems to overcome, such as the regeneration of adsorbent, the organism activity, high energy consumption, or higher costs. Thus, the economical and efficient approaches to removing p-nitrophenol from aqueous solutions are highly desired. © XXXX American Chemical Society

Reactive extraction, based upon the reversible reaction of the specific functional groups of solute and extractant, is a promising separation technique for organic compounds with high efficiency and selectivity. Recently, it has been widely used for the separations of carboxylic acids, 28−33 amines, 34 phenols,35,36 alcohol,37,38 and amphoteric compounds,39−42 and a series of encouraging results have been received. As a typical polar organic compound, p-nitrophenol has a functional group of −OH, which enables them to react with Lewis-base extractants thus being extracted into organic phases. Therefore, it has great promise to separate PNP from aqueous solutions using a reactive extraction technique. However, as far as our knowledge, there are few works reported on separating PNP using this technique. In the present work, the reactive extraction of PNP using TBP as extractant and solvent naphtha or n-octanol as diluents was investigated. The back-extraction was conducted using sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) as stripping agents. The influences of equilibrium pH (pHeq), initial TBP, and PNP concentrations on distribution ratio (D) and degree of extraction (%E) were discussed. On the basis of the experimental data, an expression of the extraction equilibrium was proposed and the parameters were also calculated. By comparing the predicted D from the model, the applicability of the proposed model was validated. Received: July 23, 2015 Accepted: January 19, 2016

A

DOI: 10.1021/acs.jced.5b00636 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION 2.1. Chemicals. p-Nitrophenol (PNP) with the purity of ≥99.5% and density of 1.495 g·cm−3 is a light yellow powder and is purchased from Beijing Chemical Reagent Co., Ltd. Tributylphosphate (TBP) with volume fraction of ≥98% and an average molecular weight of 266 kg·kmol−1 and a density of 0.976 g·cm−3 was received from Henan Yanshi Oda Chemical Reagent Co., Ltd. solvent naphtha with the density of 0.789 g· cm−3 was also obtained from Henan Yanshi Oda Chemical Reagent Co., Ltd. The components of solvent naphtha were reported as a mixture (C5−C7) of alkyl (88.17%), naphthenic hydrocarbon (7.53%), and aromatic hydrocarbon (4.3%). The n-octanol, analytical grade, with the purity of ≥99.0% and the density of 0.827 g·cm−3 was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. 2.2. Experimental Procedure. The aqueous phase was PNP dissolved in water with a concentration of 5 mmol·kg−1. The organic phases were pure solvent naphtha, n-octanol, TBP, or TBP in solvent naphtha or n-octanol with the concentrations of 0.3628, 0.7257, 1.090, and 1.451 mol·L−1. All extraction experiments were conducted in 100 mL Erlenmeyer flasks by shaking the equal volumes (15 mL) of initial aqueous and organic phases at 25 ± 0.5 °C. This process would last for 90 min in a HZQ-F160A constant temperature water bath-vibrator (Shanghai Yiheng Medical Instruments Factory) at 180 ± 10 rpm. Then the mixture was allowed to stand for 120 min to achieve complete separation. Before and after the extraction experiment, the initial and equilibrium pH of aqueous solutions were measured by a HI1200B composite electrode using a pH meter (HANNA pH 211 Microprocessor pH meter, Italy) with ±0.01pH. The relative uncertainty in the concentration measurement was found to be within ±1%. The uncertainty in the experimental results was found to be within ±3% error. The concentration of PNP in aqueous phase was determined on a UV−vis spectrophotometer (Shimadzu UV1750, Shimadzu Business Management Co., Ltd.) at 317 nm (maximum adsorption wavelength of PNP under selected conditions of pH = 3.0). The typical absorption spectrum and calibration curves are shown in Figure 1. The calibration curve

organic phase into the sample cell with a thickness of 0.05 mm between two KBr plates in the range of 4000−400 cm−1.

3. RESULTS AND DISCUSSION 3.1. Extraction Equilibrium Results. A concentration of 5 mmol·kg−1 PNP was equilibrated with various TBP/solvent naphtha or n-octanol concentration under the different pHeq values. The results are presented in Tables 1 and 2 and Figures Table 1. Extraction Equilibrium Results for the PNP + TBP/ solvent naphtha System total conc of PNP/ mmol·kg−1

Figure 1. Adsorption spectra of PNP.

equation was calculated as A = 9.405c + 0.01445 (correlation coefficient, 0.9999; range of concentrations, 0.003595−0.2517 mmol·kg−1), where A is the absorbance and c represents the concentration of PNP in aqueous solution. The loading of PNP in organic phases was calculated by material balance. The deviation of this method was less than 3%. The FT-IR spectra were measured on a VERTEX 70 (Step-Scan: 5 ns time resolution, Bruker, Germany) IR spectrometer by injecting the

init conc of TBP mol·L−1

init/equil pH in aq phase

in aq phase

in org phase

%E

D

0 0 0 0 0 0 0 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 1.090 1.090 1.090 1.090 1.090 1.090 1.090 1.090 1.451 1.451 1.451 1.451 1.451 1.451 1.451 1.451

1.21/1.68 1.33/2.05 1.67/2.75 2.55/3.13 3.13/3.87 3.67/4.88 5.77/6.57 1.34/1.73 1.56/2.20 1.98/2.64 2.44/3.04 3.10/3.64 3.95/4.73 5.53/6.25 7.61/8.10 1.22/1.79 1.72/2.51 2.14/3.12 2.95/3.89 3.30/4.56 4.92/5.56 5.58/6.87 7.23/8.56 1.21/1.71 1.56/2.45 2.13/3.22 3.12/3.99 3.32/4.75 4.67/5.78 6.91/7.12 7.34/8.34 1.10/1.56 1.43/2.15 2.18/2.98 2.53/3.24 3.54/4.12 5.67/6.12 6.18/7.05 7.50/8.04

0.1341 0.1241 0.1219 0.1323 0.1462 0.1563 0.2232 0.03333 0.03185 0.03246 0.03290 0.03311 0.03472 0.05319 0.6250 0.01400 0.01344 0.01381 0.01441 0.01445 0.01613 0.02660 0.1666 0.008957 0.008547 0.009331 0.009942 0.01057 0.01116 0.02304 0.08770 0.006024 0.005910 0.006142 0.006226 0.006536 0.006953 0.01193 0.02119

4.866 4.876 4.878 4.876 4.854 4.844 4.777 4.967 4.968 4.967 4.967 4.967 4.965 4.946 4.375 4.986 4.986 4.986 4.985 4.985 4.984 4.973 4.833 4.991 4.991 4.990 4.990 4.989 4.988 4.977 4.912 4.994 4.994 4.993 4.993 4.993 4.992 4.987 4.979

97.32 97.52 97.56 97.52 97.08 96.88 95.54 99.34 99.36 99.34 99.34 99.34 99.30 98.92 87.50 99.72 99.72 99.72 99.70 99.70 99.68 99.46 96.66 99.82 99.82 99.80 99.80 99.78 99.76 99.54 98.24 99.88 99.88 99.86 99.86 99.86 99.84 99.74 99.58

36.3 39.3 40.0 36.8 33.2 31.0 21.4 149 156 153 151 150 143 93.0 7.00 356 371 361 346 345 309 187 29.0 557 584 535 502 472 447 216 56.0 829 845 813 802 764 718 418 235

2 and 3. It can be seen that the increase of TBP concentration leads to an increase in D values. As the TBP concentration increases, the extraction equilibrium moves toward the direction that forms the extraction complex. This phenomenon could be easily explained by taking into account Le Chatelier’s principle. As known, PNP has a typical Lewis acid group hydroxyl, which makes it react with the Lewis base group B

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Table 2. Extraction Equilibrium Results for the PNP + TBP/n-Octanol System total conc of PNP/mmol·kg−1 init conc of TBP mol·L 0 0 0 0 0 0 0 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.3628 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 0.7257 1.090 1.090 1.090 1.090 1.090 1.090 1.090 1.090 1.451 1.451 1.451 1.451 1.451 1.451 1.451 1.451

−1

init/equil pH in aq phase

in aq phase

in org phase

%E

D

1.12/1.75 1.67/2.07 2.33/2.89 3.12/3.23 3.87/4.12 4.10/5.61 6.88/7.89 1.13/1.72 1.90/2.10 2.34/2.65 2.78/3.03 2.97/3.62 3.59/4.71 5.72/6.21 7.48/8.20 1.24/1.81 1.87/2.45 3.41/3.91 3.86/4.32 5.13/5.64 6.35/6.97 7.23/7.82 8.10/8.90 1.13/1.83 1.85/2.23 2.89/3.45 3.37/4.18 4.52/5.34 6.11/6.65 6.81/7.56 7.35/8.67 1.32/1.70 2.35/2.73 2.59/3.21 2.99/3.56 4.28/4.67 4.52/5.21 5.10/6.45 6.73/7.51

0.06542 0.06452 0.05025 0.05974 0.06460 0.07886 0.08288 0.03786 0.03648 0.03729 0.03729 0.03858 0.05256 0.3333 2.500 0.02233 0.02058 0.02464 0.02645 0.03704 0.05376 0.1190 0.3571 0.01420 0.01385 0.01563 0.01724 0.01969 0.02050 0.04716 0.2000 0.01190 0.01173 0.01282 0.01333 0.01401 0.01471 0.01672 0.05050

4.935 4.936 4.950 4.940 4.936 4.921 4.917 4.962 4.964 4.963 4.963 4.962 4.947 4.667 2.499 4.977 4.980 4.975 4.974 4.963 4.946 4.880 4.642 4.986 4.986 4.985 4.983 4.980 4.980 4.952 4.800 4.988 4.988 4.987 4.987 4.986 4.985 4.983 4.949

98.70 98.72 99.00 98.80 98.72 98.42 98.34 99.24 99.28 99.26 99.26 99.24 98.94 93.34 49.98 99.54 99.60 99.50 99.48 99.26 98.92 97.60 92.84 99.72 99.72 99.70 99.66 99.60 99.60 99.04 96.00 99.76 99.76 99.74 99.74 99.72 99.70 99.66 98.98

76.2 76.5 98.5 82.7 76.4 62.4 59.3 131 136 133 133 132 128 14.0 1.00 223 242 202 188 134 92.0 41.0 13.0 351 360 319 289 253 243 105 24.0 419 425 389 374 356 339 298 98.0

Figure 2. D or Dcal versus pHeq of PNP + TBP/solvent naphtha system. (Scattered point and solid line represents D and Dcal, respectively.)

Figure 3. D or Dcal versus pHeq of PNP + TBP/n-octanol system. (Scattered point and solid line represents D and Dcal respectively.)

phosphoryl of the TBP through a hydrogen bond to form complexes. The reaction could be considered as a reversible balance. With the increase of TBP concentrations, the

extraction equilibrium moves toward the direction of the formation of complexes, which makes the D value larger. C

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From Tables 1 and 2 and Figures 2 and 3, it can also be seen that D and E values are highly dependent on the values of pHeq. Also, D values vary with the increase of pHeq and the highest D values always appear at pHeq of 2−5.5. As the values of pHeq increases, D values first increase and then decrease and appear as a maximum value with the various TBP concentrations. This trend of D versus pHeq is similar to that of the molar fraction of neutral PNP in aqueous solution, indicating that efficient extraction of PNP using TBP mainly occurs between TBP and a neutral PNP molecule. The effects of various diluents on the extraction ability of solvent naphtha and n-octanol for PNP was systematically investigated. From Figures 2 and 3, it can be seen that the noctanol has a better physical extraction ability than solvent naphtha according to the theory of “similarity and intermiscibility”. Because the PNP and n-octanol belong to polar compounds. Nonpolar complexes are apt to dissolve by stronger Van Edward force. As seen from Figure 4, PNP solutions with different initial concentrations were examined by extraction with 1.451 mol·L−1

Figure 5. IR spectra of PNP + TBP/solvent naphtha. pHeq: 1, 1.24; 2, 6.58; 3, 9.97.

Figure 6. IR spectra of PNP + TBP/n-octanol. pHeq: 1, 1.24; 2, 5.21; 3, 8.36.

Figure 4. D versus initial concentration of PNP. Red circle, 1.451 mol· L−1 TBP/solvent naphtha (pHin = 2.71 ± 0.02 ; pHeq = 2.8 ± 0.02); black square, 1.451 mol·L−1 TBP/n-octanol (pHin = 2.71 ± 0.02; pHeq = 2.8 ± 0.05).

TBP/solvent naphtha or n-octanol. It is obvious that D values slightly decreased with the increase of initial concentration of PNP. This might be attributed to that the gradual increase of PNP concentration made the extractant’s loading solute close to stoichiomentric saturation. By taking into account the principle of chemical balance motion, this phenomenon could be easily explained. However, D values are still greater. The result proves that the extraction system is very effective whatever the concentration of PNP is. 3.2. IR Spectra Analysis. In order to investigate the interaction between PNP and TBP, IR spectra of the loading organic phases were recorded, as well as PNP and original organic phase. Figures 5 and 6 show the IR spectra of TBP/ solvent naphtha or n-octanol loading with PNP at different pHeq values. It has no significant difference, indicating that pHeq had an ignorable influence on the structure of complexes. Infrared spectra of TBP/solvent naphtha loading with PNP are recorded on an infrared spectrometer as well as the original TBP/solvent naphtha and pure PNP (Figure 7). TBP is a typical polar solvent containing functional group PO, which makes it interact with other molecules through hydrogen bonding action. As can be seen from the Figure 7, the band at 1278.7 cm−1 is the characteristic stretching of PO in the TBP/solvent naphtha. The new band was shown with a slight

Figure 7. IR spectra of 1, PNP; 2, TBP/solvent naphtha, and 3, PNP + TBP/solvent naphtha.

shift toward lower wavenumbers at 1267.1 cm−1 in loading PNP. It indicates that the interactions between TBP and PNP are based upon the hydrogen bond between PO of TBP and OH of PNP. Infrared spectra of TBP/n-octanol loading with PNP are recorded on an infrared spectrometer as well as the pure TBP, the original TBP/n-octanol, and pure PNP (Figure 8). The band at 1276.8 cm−1 is the characteristic stretching of PO in pure TBP. The new band was shown with a shift toward lower wavenumbers at 1257.5 cm−1 in the original TBP/n-octanol. The new band at 1257.5 cm−1 is the characteristic stretching of PO in the original TBP/n-octanol whereas the band still at 1257.5 cm−1 is the characteristic stretching of PO in loading PNP. It proves the interactions between hydroxyl in n-octanol and phosphoryl in TBP through a hydrogen bond. The noctanol concentrations is far higher than that of PNP, resulting D

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where φ is the volume fraction of solvent naphtha or n-octanol in organic phase. The value of φ is from 0.6 to 0.9. Then m is the physical extraction equilibrium constants are close to 34 and 76 in solvent naphtha or n-octanol, respectively. Furthermore, estimates of the initial TBP concentration is B0, which is far higher than PNP, and we can regard that the TBP concentration has almost no change before and after extraction. So, eq 4 can be revised as follows D=

(KB0n + φm) (1 + 10 pH − pKa)

(5)

Linearizing eq 5 by taking the logarithm on both sides, we get eq 6

Figure 8. IR spectra of 1, TBP; 2, PNP; 3, TBP/n-octanol, and 4, PNP + TBP/n-octanol.

log[D(1 + 10 pH − pKa) − φm] = log K + n log B0

(6)

According to eq 6, the plot of log[D(1 + 10 ) − φm] versus log B0 would yield a straight line with a slope of n, as shown in Figure 9. The slopes, n, for TBP/solvent naphtha or pH‑pKa

in the fact that the forming complexes between PNP and TBP by hydrogen bond was covered. 3.3. Description of Extraction Equilibrium. The reactive extraction of PNP with TBP in solvent naphtha or n-octanol could be governed by the mass action law.43 The overall extraction consists of reactive extraction by TBP and physical extraction by solvent naphtha or n-octanol. The reactive extraction occurs by means of forming a hydroxyl-bond between hydroxyl in PNP and phosphoryl in TBP at the interface between aqueous and organic phases. If the activities of all species are assumed to be proportional to their concentrations, and the reactive extraction and physical extraction are accorded with simple addition, then the reactive extraction of PNP (marked as HA in following equations) by TBP is

Figure 9. log [D(10pH−pKa) − φm] versus log B0..

K

HA + nTBP ↔ HA·TBPn

n-octanol are calculated to be 1.18 and 0.91, which are close to 1, indicating one TBP molecule reacts with one PNP molecule in the extraction process. To get the K value, take n = 1 into eq 5. The K is shown in Figure 10 and K = 426.3 and 249.8 L· mol −1 under the TBP/solvent naphtha or n-octanol, respectively.

The physical extraction of PNP by solvent naphtha or noctanol is m

HA ↔ HA

with the equilibrium constants [HA·TBPn]

K=

n

m=

[HA]·[TBP]

(1)

HA HA

(2)

where K is the apparent reactive extraction equilibrium constant, m is the physical extraction equilibrium constant, n is the number of TBP molecule per complexes, and the overbar denotes species in the organic phase. PNP dissociates in aqueous phase is given as follows Ka

HA ↔ A− + H+ Figure 10. D(1 + 10pH−pKa) − φm versus B0.

where the dissociation balance constant is Ka =

[H+][A−] [HA]

According to B0, m, pHeq, and pKa, as well as n and k were calculated and the theoretical D values for the two systems could be calculated, marked as Dcal. The smooth solid lines in Figures 2 and 3 were plotted with Dcal versus pHeq. The scattered points represent experimental D values. Figures 2 and 3 demonstrate an acceptable deviation range from experimental D values. This proves that the fit is in agreement with the

(3)

with pKa value of PNP is 7.15. According to eqs 1−3, the experimentally accessible D values can be calculated as D=

[HA·TBPn] + [HA] (K[TBP]n + φm) = [HA] + [A−] (1 + 10 pH − pKa)

(4) E

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theoretical foundation and reference data for the wastewater treatment of PNP and would supply valuable information for the development and promotion of reactive extractions.

experimental value. Also, the proposed model is feasible and validated to the discussed extraction systems.

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4. BACK-EXTRACTION The extractant (TBP, solvent naphtha) needed to be regenerated by selecting stripping agents. Therefore, NaOH and Na2CO3 solutions with various concentrations were selected as stripping agents to attempt to recover PNP. The back-extraction experiments were conducted after extraction of 5 mmol·kg−1 PNP by TBP in solvent naphtha or n-octanol and the results are shown in Figure 11, given as the plots of the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 371 23881589. Funding

The work is supported by The People’s Government of Henan Province, China (Nos. 132300410237, 134300510030), and Program for He’nan Innovative Research Team in University (15IRTSTHN005). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 11. E% versus a different stripping agent and concentration of back-extraction for the PNP. Red square, TBP/solvent naphtha; blue square, TBP/n-octanol system.

recovery ratios PNP. NaOH had a superior back-extraction rate than Na2CO3. It might be attributed to the stronger alkalinity of NaOH than that of Na2CO3. The NaOH could supply a much higher concentration of OH−, which not only could react with PNP to form negative ions that transfer from the organic phase to the aqueous phase but also could react with the phosphoryl of TBP via its H terminal, resulting in a positive effect on backextraction while CO32− could not. Figure 11 shows the backextraction rate that 99.9% PNP can be recovered by using 50 mmol·kg−1 NaOH in TBP/solvent naphtha, and 99.2% PNP can be recovered by using 50 mmol·kg−1 NaOH in TBP/noctanol. Obviously NaOH shows satisfactory results in TBP/ solvent naphtha extraction system.

5. CONCLUSION The extraction of PNP by using TBP dissolved in solvent naphtha and n-octanol was investigated. The D values were found to be highly dependent on equilibrium pH values and the initial concentration of TBP in solvent naphtha and n-octanol. The optimum operation conditions were TBP/solvent naphtha at the equilibrium pH of 2−5.5. The D values increased with the raising of the TBP concentrations. The D decreased slightly with the increase of initial concentration of PNP. TBP mainly reacted with neutral PNP, resulting in the formation of 1:1 complexes through a hydrogen bond between PO of TBP and OH of PNP, which was confirmed by IR spectra analysis. Also, nonpolar hydrogen-bond association can be much more stable in the presence of inert solvent naphtha. Furthermore, the back-extraction rate could be achieved at 99.9% using the 50 mmol·kg−1 NaOH. The results proposed a significant F

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DOI: 10.1021/acs.jced.5b00636 J. Chem. Eng. Data XXXX, XXX, XXX−XXX