Quaternary Pyridinium Ketoximes as Zinc Extractants from Chloride

Article pubs.acs.org/jced

Quaternary Pyridinium Ketoximes as Zinc Extractants from Chloride Solutions Karolina Wieszczycka,* Aleksandra Wojciechowska, Marta Krupa, and Roksana Kordala-Markiewicz Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland ABSTRACT: The goal of this work was to study the extraction of zinc(II) ions from chloride solutions with the hydrophobic quaternary 3-pyridinium ketoximes. The influence of pH, contact time, concentration of chloride ions and extractant, as well as the effect of ligand structure on the zinc(II) extraction were investigated. The studies showed that at 4 M Cl−, regardless of the acidity of the aqueous chloride solutions and type of anions attached to pyridinium ring, the zinc(II) extraction increased with increasing length of the alkyl chain. Furthermore, the extraction from a multielemental solution is selective and the created complex can be decomposed after being shaken with the aqueous solutions of sodium sulfate or sodium hydroxide. It was also indicated that the zinc(II) extraction by the hydrophobic quaternary 3-pyridinium ketoximes proceeded via an ion-exchange mechanism, and the molar ratio of zinc to ligand to chloride in the complex molecule was 1:1:2, 1:1:3, and 1:2:2.

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analysis of the chloride ions concentration and by fitting the theoretical with the experimental values.

INTRODUCTION Hydrophobic pyridylketoximes have been proposed as ligands for copper(II), zinc(II), cadmium(II), and iron(III) removal using a liquid−liquid extraction technique.1−7 The results showed the influence of a substituent position at the pyridine ring, composition of an aqueous feed and type of the organic phase modifier on the extraction process. The recent studies indicated that the 3-pyridylketoxime containing the decyl alkyl chain is the effective extractant of zinc(II) ions from the hydrochloric acid solutions and, in the presence of other metals such as copper(II) or iron(II) and iron(III), the process is very selective.7 The following disadvantages of the process should be pointed out: the efficiency of the zinc extraction process by the 3-pirydylketoxime depends mainly on the contact area, contact time of the two phases and the concentration of a protonated form of the ligand, which is formed in the initial stage of the extraction. Moreover, the protonated form of the ligand decomposes after contact with the stripping agent and the next extraction stage also requires a high concentration of hydrochloric acid in the aqueous feed. These problems can be overcome by the structure modification from the 3pyridineketoxime to the quaternary 3-pyridinium ketoxime. The goal of the research was to study the extraction ability of the hydrophobic quaternary 3-pyridinium ketoximes toward zinc(II) ions from weak and strong acidic solutions. The influence of pH, contact time, concentration of chloride ions and the extractant, as well as the effect of the ligand structure on the zinc(II) extraction were studied. The stoichiometry of the extracted species and equilibrium reaction of extraction were predicted from the slope analysis, confirmed by the © 2013 American Chemical Society

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EXPERIMENTAL SECTION Reagents. The quaternary pyridinium ketoximes were synthesized in a three stage reaction (Scheme 1). In the first stage 1-(3-pyridyl)undecan-1-one was synthesized by treating 3-pyridylcarbonitrile with decylmagnesium bromide. In the second stage, the synthesized ketone was treated with hydroxylamine hydrochloride in the presence of sodium carbonate (at pH 7). After hot filtration and cooling, yellow Scheme 1. Synthesis of Quaternary Pyridinium Ketoximes

Received: July 10, 2013 Accepted: September 30, 2013 Published: October 11, 2013 3207

dx.doi.org/10.1021/je400646z | J. Chem. Eng. Data 2013, 58, 3207−3215

Journal of Chemical & Engineering Data

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activity of water. The extraction was carried out from solutions containing variable chloride ion concentrations (0 to 4 mol· L−1), at constant water activity (aw = 0.835) and at a constant total concentration of molecules dissolved in the aqueous solution (σ = 8.0 mol·L−1) or from solutions containing a constant concentration of hydrogen cation (0.5 mol·L−1), at variable chloride ion concentrations (0 to 4 mol·L−1) and at constant value of σ = 8.0 mol·L−1 and aw = 0.875. The selectivity test was done using aqueous feed containing (a) 5 g·L−1 Zn(II), 5 g·L−1 Fe(II), and 5 g·L−1 Fe(III), 0.5 or 4 mol·L−1 HCl at 4 mol·L−1 Cl−, or (b) 5 g·L−1 Zn(II), 5 g·L−1 Cu(II), 0.5 or 4 mol·L−1HCl at 4 mol Cl−·L−1. The organic phase used in the extraction studies contained the quaternary pyridinium ketoxime (0.02 to 0.2 mol·L−1) and toluene as a diluent with a 10% (v/v) addition of decan-1-ol. The metals concentrations were determined by atomic absorption spectrometry using a Z8200 (Hitachi) apparatus. The content of Fe(II) and Fe(III) was also determined by titration with K2Cr2O7 (Fe(III) was reduced to Fe(II) with 5% solution of SnCl2). Potentiometric titration (Mettler Toledo Titrator T50) was used for the determination of chloride and hydrochloric acid concentrations in the aqueous phase (titrant: AgNO3 for determination of chloride ions and NaOH for determination of the hydrogen ion concentration). The hydrochloric acid concentration in the organic phase was determined by titration in the ethanol solution: 0.1 mL of organic phase with 1 mL of indicator (0.3% bromthymol blue in ethanol) and 20 mL of ethanol were titrated by 0.005 mol· L−1 solution of sodium hydroxide until the indicator color changed from yellow to first green.9 All experiments were performed in duplicate to ensure reproducibility of Zn(II), Fe(III), and Fe(II) concentration. The result agreed to within 1 % for Zn and 1.8 % for iron ions. The potentiometric titrations were repeated three times and an average consumption of the titrant was calculated. The average standard deviation for Cl− was 1.5 %, for H+ it was 1 % and 4 % in aqueous and organic solutions, respectively. 2.4. Calculation. The metal content in the organic phase was determined by a mass balance between the concentration of a metal in the aqueous phases before and after extraction. Distribution coefficient (D), separation factor (Szn(II)/Me) and percentage extraction (% E) were calculated from equations:

crystals were obtained. Recrystallization from ethanol and next from petroleum ether yielded pure (99.8%) yellow crystals melting at 397.7 K to 398.5 K. In the third stage, the synthesized oxime was stirred and heated (333 K) for 12 h with appropriate alkyl halides in ethanol as diluent to give quaternary pyridinium ketoximes.8 The yields of the final products were 50 % to 63 %. NMR (1H, 13C) spectras proved the structure of synthesized compounds. 1-Methyl-3-[1-(hydroxyimino)undecyl]pyridinium iodide. 1 H NMR (CDCl3) δ in ppm: 9.17 (s); 8.86 (s); 8.55 (d); 8.06 (t); 7.32 (d); 4.59 (s); 2.74 (t); 1.50 (q); 1.31−1.22 (m); 0.84 (t). 13C NMR (CDCl3) δ in ppm: 156.7; 149.0; 147.1; 133.9; 132.1; 123.5; 50,1; 31.7; 29.7; 29.4; 29.2; 29.1; 26.1; 26.0; 25.9; 22.5; 13.9. 1-Ethyl-3-[1-(hydroxyimino)undecyl]pyridinium bromide. 1 H NMR (CDCl3) δ in ppm: 9.19 (s); 8.80 (s); 8.55 (d); 8.06 (t); 7.32 (d); 3.65 (t); 2.74 (t); 1.50 (q); 1.31−1.19 (m); 0.82 (m). 13C NMR (CDCl3) δ in ppm: 156.4; 148.9; 147.3; 133.8; 132.2; 122.9; 62.5; 32.7; 31.9; 29.8; 29.7; 29.4; 29.1; 26.2; 25.7; 22.7; 18.1; 14.1. 3-[1-(Hydroxyimino)undecyl]-1-propylpyridinium bromide. 1H NMR (CDCl3) δ in ppm: 9.22 (s); 8.90 (s); 8.50 (d); 7.90 (t); 7.29 (d); 3.60 (t); 2.78 (t); 1.51 (q); 1.31−1.19 (m); 0.81 (m). 13C NMR (CDCl3) δ in ppm: 156.2; 148.9; 147.1; 133.8; 132.3; 123.4; 62.5; 32.6; 31.7; 29.7; 29.5; 29.4; 29.1; 26.1; 25.7; 22.5; 18.1; 14.0; 10.3. 3-[1-(Hydroxyimino)undecyl]-1-propylpyridinium chloride. 1H NMR (CDCl3) δ in ppm: 9.15 (s); 8.88 (s); 8.22 (d); 7.90 (t); 7.41 (d); 3.61 (t); 2.96 (t); 1.54 (q); 1.31−1.24 (m); 0.86 (m). 13C NMR (CDCl3) δ in ppm: 156.5; 148.7; 147.4; 133.6; 132.3; 123.6; 62.8; 32.7; 31.9; 29.7; 29.5; 29.4; 29.3; 26.2; 25.7; 23.7; 22.8; 18.3; 14.0. 1-Butyl-3-[1-(hydroxyimino)undecyl]pyridinium bromide. 1 H NMR (CDCl3) δ in ppm: 9.15 (s); 8.88 (s); 8.22 (d); 7.90 (t); 7.41 (d); 3.61 (t); 2.96 (t); 1.54 (q); 1.31−1.24 (m); 0.86 (m). 13C NMR (CDCl3) δ in ppm: 156.2; 148.7; 147.3; 133.7; 132.3; 123.4; 62.6; 32.6; 31.8; 29.7; 29.5; 29.4; 29.2; 26.2; 25.5; 23.4; 22.2; 18.2; 19.4, 13.6. 3-[1-(Hydroxyimino)undecyl]-1-pentylpyridinium bromide. 1H NMR (CDCl3) δ in ppm: 9.19 (s); 8.86 (s); 8.52 (d); 7.90 (t); 7.29 (d); 3.59 (t); 2.78(t); 1.52 (q); 1.31−1.22 (m); 0.84−0.76 (m). 13C NMR (CDCl3) δ in ppm: 156.1; 148.8; 147.0; 133.8; 132.3; 123.3; 62.5; 32.5; 31.7; 29.7; 29.5; 29.4; 29.1; 26.1; 25.6; 25.5; 22.5; 21.9; 18.1; 13.9; 13.6. All reagents used in this study were of reagent grade. Toluene (99.8 %, POCH, Poland) and decan-1-ol (> 99 %, Merck, Germany) were used as components of the organic phase. Sodium chloride (ACS reagent, Sigma-Aldrich, Germany), sodium nitrate(V) (ACS reagent, Sigma-Aldrich, Germany), hydrochloric acid (38 %) (AR reagent, POCH, Poland), nitric acid (70 %) (AR reagent, POCH, Poland), zinc(II) chloride (anhydrous) (ACS reagent, Sigma-Aldrich, Germany) and zinc(II) nitrate (hexahydrate) (ACS reagent, Sigma-Aldrich, Germany) were used to compose the aqueous phase. Procedure. Basic extraction and stripping studies were carried out in a test tube using an organic to aqueous volume ratio (O/W) equal to 1. Both phases were shaken for 30 min at room temperature (294 to 296 K) using Biomix BWR 04. Aqueous feed solutions were prepared by dissolving appropriate amounts of chloride or nitrate salts of zinc(II) in ultrapure water. NaCl, LiNO3, and NaNO3 were used to adjust the

D=

[metal]org [metal]aq

where [metal]aq and [metal]org are the metal concentration after the extraction in the aqueous and organic phases, respectively; SZn(II)/Me =

DZn DMe

where Me represents Cu(II), Fe(II), or Fe(III) ions.

%E =

D × 100 Vaq ⎞ ⎛ ⎜D + ⎟ Vorg ⎠ ⎝

where Vaq and Vorg are the volumes of the aqueous and organic phases, respectively.

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RESULTS AND DISCUSSION Extraction Studies. Effect of pH. The effect of pH on the zinc extraction with 3-[1-(hydroxyimino)undecyl]-1-propyl3208



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pyridinium chloride and 3-[1-(hydroxyimino)undecyl]-1propylpyridinium bromide dissolved in toluene with 10 % (v/ v) of decan-1-ol was studied using the aqueous solution containing 5 g·L−1 of Zn2+, 2 mol·L−1 of Cl−, and 2 mol·L−1 of NO3−. The initial pH levels (0.5, 1.0, 1.5, 2.5, 3.5 and 4.5) were adjusted by nitric acid. The results illustrated in Figure 1 show

Figure 2. Effect of the phases contact time on zinc(II) extraction with 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride (black) and 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide (gray) dissolved in toluene with 10% (v/v) addition of decan-1-ol ([Zn2+] = 5 g·L−1, [Cl−] = 4 mol·L−1, pH = 0, [LR+X−] = 0.1 mol· L−1).

Figure 1. Effect of equilibrium pH on zinc(II) extraction with 3-[1(hydroxyimino)undecyl]-1-propylpyridinium chloride (black square) and 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide (gray square) dissolved in toluene with 10 % (v/v) addition of decan-1-ol ([Zn2+] = 5 g·L−1, [Cl−] = 2 mol·L−1, [RL+X−] = 0.1 mol·L−1).

that in the case of 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide the extraction does not depend on the equilibrium pH. But, for 3-[1-(hydroxyimino)undecyl]-1propylpyridinium chloride a 3 % decrease in the extraction efficiency under acidic conditions was observed (the decrease of the extraction from 12 % at the initial pH = 1 to 9 % at pH = 0.5). The negligible effect of the pH on the zinc extraction may suggest the metal coordination only by the pyridinium group through an ion-exchange as well as an ion-pair mechanism. Effect of Phases Contact Time. The effect of the phases contact time on zinc(II) extraction was also examined using 3[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride and bromide as ligands and two different aqueous solutions: one of these solutions contained 5 g·L−1 of Zn(II), 2 mol·L−1of Cl−, and 0.5 mol·L−1 of H+, and the other contained 5 g·L−1 of Zn(II), 2 mol·L−1 of Cl−, with a pH of 4.5. The phase contact time was varied from 5 to 60 min. The obtained results displayed in Figure 2 show that, regardless of the pH of the aqueous phase and the structure of the ligand, the optimum shaking time was 15 min. Effect of Ligand Structure. The effect of the ligand structure on zinc(II) extraction was examined using the following ligands: 1-methyl-3-[1-(hydroxyimino)undecyl]pyridinium iodide, 1-ethyl-3-[1-(hydroxyl-imino)undecyl]pyridinium bromide, 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide, 1-butyl-3-[1-(hydroxyimino)undecyl]pyridinium bromide and 3-[1-(hydroxyimino)-undecyl]-1-pentylpyridinium bromide. The extraction process was carried out with the aqueous solutions containing 5 g·L−1 Zn2+ and the following: (a) 0.5 mol·L−1 Cl− and 3.5 mol·L−1 NO3− (I = 4 mol·L−1, aw = 0.872) at pH = 0 (b) 0.5 mol·L−1 Cl− and 3.5 mol·L−1 NO3− (I = 4 mol·L−1, aw = 0.853) at pH = 4.5 (c) 4 mol·L−1Cl− (I = 4 mol·L−1, aw = 0.875) at pH = 0 (d) 4 mol·L−1Cl− (I = 4 mol·L−1, aw = 0.853) at pH = 4.5 The results presented in Figure 3 show that the extraction of zinc(II) ions from the chloride solution depends strongly on

Figure 3. Influence of alkyl chain length attached to pyridinium ring on extracion of Zn(II) ions with the quaternary pyridinium ketoximes: [Zn2+] = 5 g·L−1; [LR+X−] = 0.1 mol·L−1: light gray, 0.5 mol·L−1 Cl−, pH = 0; medium gray, 0.5 mol·L−1 Cl−, pH = 4.5; dark gray, 4 mol· L−1Cl−, pH = 4.5; black, 4 mol·L−1 Cl−, pH = 0.

the structure of the alkyl halide attached to the pyridinium nitrogen atom and the composition of the aqueous phase. In the case of the extraction from the acidic aqueous solution containing 0.5 mol·L−1 of Cl−, regardless of the ligands structure, very low efficiency of the extraction was observed. Especially, the compounds containing a methyl and an ethyl group attached to the pyridinium nitrogen atom demonstrated a particularly weak extraction of zinc(II) ions (D > 0.02). At pH = 4.5, the ligands containing a propyl, butyl, and a pentyl chain showed a comparable extraction behavior (D range 0.28 to 0.29). Similar efficiency was also observed for the compound containing the methyl group; however, in that case the effect resulted in the iodide hydration radii, which is lower than that of the bromide and chloride and easily undergoes an ionexchange with zinc-chloride anions.10 On the other hand, the extraction of zinc(II) ions from the acidic solution is less efficient than that at pH of 4.5. This fact may be explained by a competition between an ion-exchange and an ion-pair reaction,11 therefore, the amount of the extracted zinc(II) ions may depend on a concentration of neutral ZnCl2. Following the increase of the chloride ions concentration in the aqueous phase from 0.5 to 4 mol·L−1 a very clear influence of the number of carbon atoms in the alkyl group on the zinc(II) distribution coefficient was observed. The results showed that, regardless of the acidity of the aqueous chloride solution and type of anion, the zinc extraction increased with 3209



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the increasing length of the alkyl chain and the maximum extraction was observed for 3-[1-(hydroxyimino)undecyl]-1pentylpyridinium bromide. The influence of the type of anion attached to the pyridinium ring was also studied; however, for this analysis 3-[1(hydroxyimino)undecyl]-1-propylpyridinium bromide and chloride were chosen. It was found that the zinc extraction strongly depended on the pyridinium anion type. The presence of a bromide ion in the structure of the quaternary pyridinium ketoxime resulted in 28 % extraction of zinc(II) ions (4 mol·L−1 Cl−, pH = 0), whereas, the presence of chloride anions enabled a less efficient extraction (12 %). Stripping. Loaded organic solutions containing 0.1 mol·L−1 of ligand (1-ethyl-3-[1-(hydroxyimino)undecyl]pyridinium bromide, 3-[1-(hydroxyimino)undecyl]-1-propyl-pyridinium bromide, 1-butyl-3-[1-(hydroxyimino)undecyl]pyridinium bromide or 3-[1-(hydroxyimino)undecyl]-1-pentylpyridinium bromide) and from 1.8 to 2.4 g·L−1of zinc ions were employed for stripping tests in which a loaded organic phase was shaken with the following: water, (0.05, 0.1 and 0.5) % of HCl, 5 % Na2SO4 and (0.05, 0.1 and 0.5) % of NaOH at phase ratio O/W equal to 1. The results indicate that regardless of the zinc extraction conditions, the stripping of zinc ions from the loaded organic phase can proceed at 100 % even after 10 min of shaking with the aqueous solutions of sodium sulfate and sodium hydroxide. However, the concentration of the sodium hydroxide solution should not be greater than 0.1 %. In the case of 1-methyl-3-[1(hydroxyimino)undecyl]pyridinium iodide only the aqueous solutions of sodium hydroxide can be used as the stripping agent, and the process proceeds at 100 %. Selective Extraction of Zinc(II) Ions over Iron(II) and Iron(III). The selective recovery of zinc(II) ions from galvanic wastewater and solid wastes is a crucial issue regarding both environmental protection and economy of the process. The zinc coating is widely employed on steel to control the corrosion process, but produced in large quantities the spent pickling solutions mainly contain FeCl2, HCl, and toxic ZnCl2.11−13 The spent pickling solutions also contain small amounts of Fe(III), which can be coextracted with zinc(II) ions.14 Therefore, the selectivity of Zn(II) extraction with the quaternary pyridinium ketoxime over Fe(II) and Fe(III) should be also determined. The selectivity of 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide dissolved in toluene with 10 % (v/v) addition of decan-1-ol was determined by contacting the organic phase with the fresh aqueous solution containing the following: (a) 5 g·L−1 Zn(II), 5 g·L−1 Fe(II), 5 g·L−1 Fe(III) and 0.5 or 4 mol·L−1 HCl; (b) 5 g·L−1 Zn(II), 5 g·L−1 Cu(II) and 0.5 or 4 mol·L−1 of HCl. The concentration of chloride ions was constant and equal to 4 mol·L−1. The studies show that, in the case of the mixture containing Zn(II), Fe(II), Fe(III) and 0.5 mol·L−1 of HCl, zinc(II) ions are extracted to the organic phase at (46 to 49) %, while Fe(II) at 100 % and Fe(III) at 99.9 % remain in the aqueous phase (Figure 4). Unfortunately, the selectivity over Fe(III) decreases with the increase of HCl concentration and for the solution containing 4 mol·L−1 HCl the parameter SZn(II)/Fe(III) decreases to just 50. Regardless of hydrochloric acid concentration Fe(II) ions are not extracted from acidic chloride solutions. Extraction Isotherm. The loaded capacity of 3-[1(hydroxyimino)undecyl]-1-propylpyridinium bromide (0.1 mol·L−1) was determined by contacting the organic phase

Figure 4. Selective extraction of zinc ions from aqueous solution containing: 5 g·L−1 Zn(II), 5 g·L−1 Fe(II), 5 g·L−1Fe(III), and 0.5 or 4 mol·L−1HCl.

with fresh aqueous solutions containing from (2 to 40) g·L−1 of Zn(II), 5 g·L−1 of Fe(II), 5 g·L−1 of Fe(III), and 2 mol·L−1 HCl at organic/aqueous (O/A) ratios of 1, 2, and 4. The results showed that the maximum degree of the extraction depended on the O/A ratio, and the maximum amount of the extracted zinc(II) ions was found to be (2.6, 5.3, and 11.9) g·L−1 at the O/A phases ratio of 1, 2, and 4, respectively. Thus, the maximum degree of the extraction is achievable at O/A = 4. The extraction isotherm and the constructed McCabe−Thiele diagram given in Figure 5 indicate that only three stages of the

Figure 5. Isotherm of zinc(II) extraction from acidic chloride solution with 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide (0.1 mol·L−1) dissolved in toluene with 10% (v/v) addition of decan-1-ol at O/A ratio = 4 (aqueous phase composition: 2 to 40 g·L−1 Zn(II), 5 g· L−1Fe(II), 5 g·L−1 Fe(III), and 2 mol·L−1 HCl).

extraction are required for the reduction of the zinc(II) ions concentration from 30 to 0.64 g·L−1 and, in the next step, zinc(II) can be stripped from the loaded organic phase using a 5 % solution of sodium sulfate at A/O phase ratio 3 and the stripping efficiency was above 93 %. Reuse of Organic Phase. The reuse of the organic phases containing 0.1 mol/L of 3-[1-(hydroxyimino)undecyl]-1propylpyridinium bromide following zinc stripping by aqueous solutions of sodium sulfate was also investigated. In that experiment, the aqueous phase contained 5 g·L−1 Zn2+, 4 mol· L−1 Cl−, and 4 mol·L−1 of HCl. The aqueous and organic phases were in contact for 15 min and then separated. The resultant organic solution was then stripped with 5 % of Na2SO4 and in the next stage, without washing, it was used for repeated processing. In that study the extraction-stripping process was repeated 20 times. The results presented in Figure 6 show that, regardless on the ligand structure, the organic phase can be used repeatedly without affecting extraction ability. 3210



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concentration of zinc(II) ions as well as the concentration of each of the zinc chlorocomplexes in the studied aqueous phase. The total concentration of zinc(II) ions in the chloride aqueous phase is equal to20 [Zn+2]T,aq = [Zn+2]aq + [ZnCl+]aq + [ZnCl 2]aq + [ZnCl3−]aq + [ZnCl4 −2]aq

(6)

where [ZnCli 2 − i]aq = βi [Zn(+2)][Cl−]i

and βi is the overall stability constant of ith zinc(II) chlorocomplexes.25−27 Thus, the equilibrium constants of reactions from eqs 1 to 5 can be expressed by15−23

Figure 6. Zinc(II) extraction with regenerated 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide (0.1 mol·L−1) dissolved in toluene with 10% (v/v) addition of decan-1-ol.

K1 = [ZnCl3−(LR+)]org [X−]aq [ZnCl3−]aq −1 [LR+X−]−1org

Extraction Mechanism. The hydrophobic extractant containing a pyridinium cation, similar to quaternary long-chain amines, can form complexes with zinc(II) ions in chloride solutions complexes through ion-pair and ion-exchange mechanisms.15−23 The application of the ligand as an ionexchange extractant should be justified with a sufficient concentration of anionic zinc chlorocomplexes, which can be ion-paired with the cation of the reagent. Furthermore, Daud and Cattral16,17 pointed to the competition between the hydrogen ion and quaternary ammonium ion for the anionic divalent metal complexes. In the studies, one of the ligand molecules was replaced by hydrogen cation forming complex of composition (R4N+)(HCdCl4−).16,17 Thus, in the case of the extraction of zinc(II) ions by the quaternary pyridinium ketoximes, the extraction reaction can be described by the following equations:15−23 ZnCl3−aq + LR+X−org = ZnCl3−(LR+)org + X−aq

(7)

(8)

or −1

K 2 = [(ZnCl3H)(LR+X−)]org [ZnCl3−]aq −1 [H+]aq [LR+X−]−1 org (9)

where [ZnCl−3 ]aq

3 β3[Zn+2]T,aq [Cl−]aq

=

4

1 + ∑i = 1 βi [Cl−]i

(10)

and −1

K3 = [(ZnCl4 −2)(LR+)2 ]org [X−]2 aq [ZnCl4 −2]aq [LR+X−]−2 org (11)

or K4 = [(ZnCl4H−)(LR+)]org [X −]aq [ZnCl4 −2]aq

(1)

−1

−1

× [H+]aq [LR+X−]−1org

ZnCl4 −2 aq + 2(LR+X−)org = (ZnCl4 −2)(LR+)2,org + 2X −aq

where

(2)

or ion replacement by a hydrogen cation: ZnCl3−aq + H+aq + LR+X−org = ZnCl3H(LR+X −)org

[ZnCl−4 2]aq

4 β4 [Zn+2]T,aq [Cl−]aq 4

1 + ∑i = 1 βi [Cl−]i

(13)

and K5 = [(ZnCl 2)(LR+)],org [ZnCl 2]aq −1 [LR+X −]−1org

(4)

(14)

where

(subscripts org and aq represent the organic and aqueous phases, respectively; LR+X− represents the molecule of the quaternary pyridinium ketoxime and X− represents the anion attached to pyridinium ring (I−, Cl−, or Br−)) The alternative mechanism, which is the ion-pair mechanism, does not require the anionic form of the metal species to be present in the aqueous phase and it implies that a neutral metal containing species is transferred to the organic phase.24 This mechanism can be represented as ZnCl 2aq + n(LR+X−)org = (ZnCl 2)(LR+X −)n ,org

=

(3)

ZnCl4 −2 aq + H+aq + LR+X−org = (ZnCl4H−)(LR+)org + X−aq

(12)

[ZnCl 2]aq =

2 β2[Zn+2]T,aq [Cl−]aq 4

1 + ∑i = 1 βi [Cl−]i

(15)

Taking into consideration the above equations, it was found that modeling of the extraction process can be achieved by analyzing the influence of the ligand, chloride, zinc(II), and the hydrogen ions concentrations in the aqueous phases. Obtained results should also be compared with the analysis of the chloride ions concentration in the aqueous phase (or bromide, if necessary) and the hydrogen ions concentration in the organic phase. To gain a better understanding of the extraction mechanism 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide and 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride were selected to be analyzed in detail. The effect of chloride ions concentration was studied in two different series. The first series were carried out from solutions

(5)

To find out the composition of the extracted species the experimental data should be treated under the assumption of the equilibrium equations. However, the assumption needs the constant values of the activity coefficients of all the molecules and species dissolved in the aqueous phase. Modeling of the extraction process also needs information about the total 3211



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Assuming a comparable activity of chloride and bromide ions28 and that the value of log βi, Zn(II), and the ligand concentrations are constant, eq 17 can be simplified to

containing variable chloride ion concentrations (0 to 4 mol· L−1), at constant water activity (aw = 0.835), at pH equal to 4.5 and at a constant total concentration of molecules dissolved in the aqueous solution (σ = 8.0 mol·L−1). NaCl, LiNO3, and NaNO3 were used to adjust the activity of water. The second series were carried out from solutions containing a constant concentration of hydrogen cation (0.5 mol·L−1) and variable chloride ion concentrations (0 to 4 mol·L−1). As in the first series, the total concentration of molecules dissolved in the aqueous solutions was equal to 8.0 mol·L−1, but the water activity of the solutions was equal to 0.875. The extraction data presented in Figure 7 show that the influence of chloride ion concentration depends not only on

4

log D + log(1 +

i=1

= log K ′ + (p − n′) log[Cl−]

where K′ = Kextβi[LR X ] , n′ = n, but, if ZnCl2 is coordinated as ZnCl2(LR+X−)n, n′ = 0. Values of the cumulative equilibrium constants of the zinc chlorocomplexes β1, β2, β3, and β4 were 1.26, 3.16, 5.01, and 0.63, respectively.27 On the basis of the above results log−log plots were constructed.20 Figure 8 shows the variation in log D +

Figure 8. Logarithmic relation between zinc(II) distribution coefficient and equilibrium chloride ions concentration for 3-[1(hydroxyimino)undecyl]-1-propylpyridinium chloride (gray square, at pH = 0; black square, at pH = 4.5) and 3-[1-(hydroxyimino)undecyl]1-propylpyridinium bromide (gray triangle, at pH = 0; black triangle, at pH = 4.5) dissolved in toluene with 10 % (v/v) addition of decan-1ol (F = log D + log(1 + Σi4= 1βi [Cl−]i); [Zn2+] = 5 g·L−1, [LR+X−] = 0.1 mol·L−1, pH = 0 or 4.5).

the extractant structure, but also on the pH of the aqueous feed. In the case of 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride, as well as, 3-[1-(hydroxyimino)undecyl]1-propylpyridinium bromide under acidic condition (pH = 0) the rate of the extraction increases linearly with the increase of the chloride ions concentration up to 2 mol·L−1and either remains fairly constant upon further addition of Cl− or decreases, but the decrease is only a few percent. The rising amount of the extracted metal is due to the increasing presence of extractable zinc(II) chlorocomplexes (mainly ZnCl3−). In the case of the extraction from weak acidic solutions (pH = 4.5) the increase of the chloride ion concentration also accelerates the process efficiently; however, nonlinearity of the curves suggests the coordination of ZnCl2 and ZnCl3− also.20 Thus, the equilibrium constant of the extraction can be generally defined as

log(1+Σi =4 1βi[Cl−]i) as a function of log[Cl−] at constant Zn(II) and the ligand concentration. Free chloride ions concentration [Cl−] was calculated from20 [Cl−] = [Cl−]T,aq − ([ZnCl+]aq + 2[ZnCl 2]aq + 3[ZnCl3−]aq + 4[ZnCl4 −2]aq )

4

(16)

log D = log K ″ + n log[LR+X−]

and

log D = log K ″ + n log[LR+X−]eq

4

− n′ log[X ] − log(1 +

− i

∑ βi [Cl i=1

(20)

and confirmed by dependence

log D = log Kext + n log[LR+X−] + log βi + (p) log[Cl−] −

(19)

As can be seen from the experimental curves in the case of both 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride and 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide the correlations are linear with slopes 2 and 3. A full description of the extraction process requires the determination of a number of ligand molecules reacting with one molecule of zinc. At constant chloride and zinc(II) ions concentrations that parameter can be predicted from the dependence20,18

[ZnCl p(LR+)n ][X−]n ′ (1 + ∑i = 1 βi [Cl−]i ) [Zn+2]βi [Cl−](p) [LR+X−]n

(18)

+ − n

Figure 7. Influence of chloride ions concetration on extracion of Zn(II) ions with 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride (gray square, at pH = 0; black square, at pH = 4.5) and 3[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide (gray triangle, at pH = 0; black triangle, at pH = 4.5) dissolved in toluene with 10% (v/v) addition of decan-1-ol ([Zn2+] = 5 g·L−1, [LR+X−] = 0.1 mol·L−1).

Kext =

∑ βi [Cl−]i )

]) (17)

(21)

where 3212


Journal of Chemical & Engineering Data K″ =

Article

Kextβi [Cl−]i 4

1 + ∑i = 1 βi [Cl−]i

(22)

+ ‑

[LR X ]eq is a free ligand concentration and is equal to the initial concentration of the ligand reduced by the complex concentration in the organic phase. The effect of the ligand concentration was studied using the organic solutions containing from 0.02 to 0.2 mol·L−1 of 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium chloride or 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide and three different aqueous solutions: (1) the aqueous phase containing 0.5 mol· L−1 Cl−, at constant water activity (aw = 0.835), at pH equal to 4.5, and at constant total concentration of molecules dissolved in the aqueous solution (σ = 8.0 mol·L−1); (2) the aqueous phase containing 4 mol·L−1 Cl−, at constant aw = 0.835 and σ = 8.0 mol/L, and at pH equal to 4.5; and (3) the aqueous phase containing 4 mol·L−1 Cl−, at constant aw = 0.875 and σ = 8.0 mol·L−1, and at pH equal to 0. The obtained results show that zinc(II) extraction increases as the concentration of the ligand increases. Furthermore, in Figure 9 and Figure 10 the experimental value of log D was

Figure 10. Logarithmic relation between zinc(II) distribution coefficient and equilibrium ligand concentration for 3-[1(hydroxyimino)undecyl]-1-propylpyridinium bromide dissolved in toluene with 10% (v/v) addition of decan-1-ol ([Zn2+] = 5 g·L−1. Symbols: gray square, 0.5 mol·L−1 Cl−, pH = 4.5; gray circle, 4 mol·L−1 Cl−, pH = 4.5; black triangle, 4 mol·L−1 Cl−, pH = 0.5).

proposed equilibrium reactions and enabled the calculation of log Kex: log Kex = log K ″ − log βi − m log[Cl−] 4

− log(1 +

∑ βi [Cl−]i ) i=1

(23)

where values of log K″ were obtained from slopes of the dependence log D = f(log[LR+X−]eq) given in Figure 9 and Figure 10. The obtained values of log K″, log Kex and propositions of complexes structures are presented in Table 1. The values of log Kex for the zinc(II) extraction with 3-[1(hydroxyimino)undecyl]-1-propylpyridinium chloride from the weak and strong acidic solutions were 0.32 ± 0.03 (0.5 mol·L−1 Cl−), 0.70 ± 0.01 (4 mol·L−1 Cl−), and at pH 0 log Kex = 1.77 ± 0.02 (4 mol·L−1 Cl−), respectively, and in the case of 3-[1(hydroxyimino)undecyl]-1-propylpyridinium bromide the values were at pH 4.5 equal to 1.54 ± 0.05 (0.5 mol·L−1 Cl−) and 2.42 ± 0.01 (4 mol·L−1 Cl−), and at pH 0 log Kex was equal to 0.65 ± 0.01. The confirmation and validation of the extraction models was conducted by fitting the experimental with the theoretical values of the distribution coefficients. The theoretical values of log D were calculated from eq 17 and from

Figure 9. Logarithmic relation between zinc(II) distribution coefficient and equilibrium ligand concentration for 3-[1(hydroxyimino)undecyl]-1-propylpyridinium chloride dissolved in toluene with 10 % (v/v) addition of decan-1-ol ([Zn2+] = 5 g·L−1. Symbols: gray square, 0.5 mol·L−1 Cl−, pH = 4.5; gray circle, 4 mol·L−1 Cl−, pH = 4.5; black triangle, 4 mol·L−1 Cl−, pH = 0.5).

plotted against log[LR+X−] and next against log[LR+X−]eq and, as can be observed, most of the obtained values give straight lines with slopes from 0.821 to 1.10; this means that zinc(II) is coordinated by one molecule of the ligand. However, in the case of 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide at pH = 4.5 and at the chloride ions concentration of 4 mol·L−1, the slope of the straight line was close to 2 (Figure 10) suggesting the coordination of zinc(II) ions by two molecules of the ligand. The slope analysis method revealed that the extraction equilibria of zinc(II) ions could be expressed by the constants K1, K2, K4, or K5 (eqs 8, 9, 12, or 14). However, the additional analysis of the chloride ions concentration, which was done for the experiments with 3-[1-(hydroxyimino)undecyl]-1-propylpyridinium confirmed that two (2.1 ± 0.5) and three (3.1 ± 0.2) molecules of chloride ions were moved to the organic phase at pH of 4.5 and 0, respectively. That confirmed the

⎛ ⎞ [Zn(II)]o ⎟ log D = log⎜⎜ − i ⎟ ⎝ ([Zn(II)]t,aq − (βi [Zn(II)][Cl ] ) ⎠

(24)

Table 2 shows the experimental value of the zinc(II) distribution coefficients together with log D calculated using eqs 17 and 24. The fit was checked by calculating an average sum of squares of a difference between calculated and experimental distribution coefficients: U=

∑ (log Dexp − log Dcalc)2

(25) N The comparison of the experimental and calculated values showed good fitting of proposed equations (Table 2). However, a greater discrepancy between the experimentally determined and calculated log D using eq 17 was observed. 3213



Article

Table 1. Values of log K″, log Kex and Proposed Complexes (QP3C-PCl) 3-[1-(Hydroxyimino)undecyl]-1-propylpyridinium Chloride and (QP3C-PBr) 3-[1-(Hydroxyimino)undecyl]-1-propylpyridinium Bromide ligand

aqueous phase composition

log K″

QP3C-PCl

0.5 mol·L−1 Cl− pH 4.5 4 mol·L−1 Cl− pH 4.5 4 mol·L−1 Cl− pH 0 0.5 mol·L−1 Cl− pH 4.5 4 mol·L−1 Cl− pH 4.5 4 mol·L−1 Cl− pH 0

−0.15 ± 0.01 −0.12 ± 0.01 0.63 ± 0.01 1.04 ± 0.01 1.61 ± 0.01 0.41 ± 0.02

QP3C-PBr

log Kex 0.32 0.70 1.77 1.54 2.42 0.65

± ± ± ± ± ±

0.03 0.01 0.02 0.05 0.01 0.01

proposed complex ZnCl3−(RL+) and/or ZnCl2(RL+X−) ZnCl4H−(RL+) ZnCl4−2(RL+)2 and/or ZnCl2(RL+X−)2 ZnCl3−(RL+) ZnCl4H−(RL+)

Table 2. Values of Experimental and Calculated log D and Values of Parameter U (U = (Σ(log Dexp − log Dcalc)2)/N); (QP3CPCl) 3-[1-(Hydroxyimino)undecyl]-1-propylpyridinium Chloride; (QP3C-PBr) 3-[1-(Hydroxyimino)undecyl]-1propylpyridinium Bromide) aqueous phase composition

experimental

U

calculated

ligand

mol·L−1 Cl−

pH

coordinated zinc(II) ion

log D ± 0.005

log D(eq 23) ± 0.038

log D (eq 17) ± 0.008

eq 23/eq 17

QP3C-PCl

0.1 0.25 0.5 1.0 2.0 3.0 4.0 0.1 0.25 0.5 1.0 2.0 3.0 4.0 0.1 0.25 0.5 1.0 2.0 3.0 4.0 0.1 0.25 0.5 1.0 2.0 3.0 4.0

4.5

ZnCl3−

ZnCl4−2

4.5

ZnCl3−

0

ZnCl4−2

−1.575 −1.045 −0.757 −0.753 −0.787 −0.803 −0.749 −3.308 −2.283 −1.704 −1.388 −1.102 −1.079 −1.150 −1.441 −1.013 −0.609 −0.422 −0.445 −0.511 −0.561 −1.418 −1.546 −1.154 −0.850 −0.537 −0.425 −0.387

−1.778 −1.139 −0.813 −0.709 −0.851 −1.019 −1.157 −9.168 −2.193 −1.535 −1.150 −0.961 −0.948 −0.961 −1.233 −0.554 −0.158 −0.027 −0.216 −0.406 −0.557 −8.804 −1.915 −1.181 −0.783 −0.675 −0.680 −0.697

0.001/0.002

0

−1.565 −1.018 −0.744 −0.689 −0.777 −0.837 −0.813 −3.308 −2.275 −1.681 −1.335 −1.001 −0.945 −0.978 −1.425 −1.018 −0.630 −0.438 −0.395 −0.439 −0.481 −1.401 −1.534 −1.126 −0.808 −0.478 −0.385 −0.387

QP3C-PBr

0.009/3.822

0.003/0.086

0.002/6.140

the chloride ions concentration, time of shaking, and the pH at low concentrations of chloride ions. The stripping of zinc(II) ions from the loaded organic phase was also investigated, and it was found that even after 10 min of shaking with aqueous solutions of sodium sulfate and sodium hydroxide, 100 % of the metal was stripped. Moreover, a regenerated organic phase can be used repeatedly in further cycles of the process, without washing, while maintaining the extraction ability. A constructed isotherm and McCabe−Thiele diagram showed that the application of phases ratio O/W = 4 at an oxime concentration of 0.1 mol·L−1 enabled a reduction of the zinc(II) ions concentration from 30 to 0.64 g·L−1 and, in the next step, zinc(II) can be stripped from the loaded organic phase using a 5 % solution of sodium sulfate at an A/O phase ratio of 3. Stoichiometry studies of the zin(II) complexes with 3-[1(hydroxyimino)undecyl]-1-propylpyridinium chloride and the

Perhaps, the equilibrium concentration of the ligands should be calculated taking into account dimerization of the quaternary pyridinium salts, as well as the other side reactions such as ionexchange reactions with other ions than Zn(II) and a transport of H+ to the organic phase.

■

CONCLUSIONS The positive effect of zinc(II) extraction from chloride solution was observed for all studied hydrophobic quaternary pyridinium ketoximes. At lower levels of aqueous phase acidity the ligands containing propyl, butyl, and pentyl alkyl chains showed comparable extraction ability. But under acidic conditions maximum extraction was observed for 3-[1(hydroxyimino)undecyl]-1-pentylpyridinium bromide at pH 0. Moreover, the extraction studies showed that the process depended not only on ligand structure, but also on ligand and 3214



Article

(13) Silva, J. E.; Paiva, A. P.; Soares, D.; Labrincha, A.; Castro, F. Solvent extraction applied to the recovery of heavy metals from galvanic sludge. J. Hazard. Mater. 2005, 120, 113−118. (14) Regel-Rosocka, M. A review on methods of regeneration of spent pickling solutions from steel processing. J. Hazard. Mater. 2010, 177, 57−69. (15) Singh, J. M.; Tandon, S. N. Extraction of zinc(II), cadmium(II), and mercury(II) oxalate complexes by high molecular weight amines and quaternary ammonium salt. J. Inorg. Nucl. Chem. 1978, 41, 897− 899. (16) Daud, H.; Cattrall, R. W. The extraction of Hg(II) from potassium iodide solutions and the extraction of Cu(II), Zn(II), and Cd(II) from hydrochloric acid solutions by Aliquat 336 dissolved in chloroform. J. Inorg. Nucl. Chem. 1981, 43, 779−785. (17) Daud, H.; Cattrall, R. W. The extraction of Cd(II) and Zn(II) from acidified lithium chloride solutions by Aliquat 336 dissolved in chloroform. J. Inorg. Nucl. Chem. 1981, 43, 599−601. (18) Sato, T.; Shimomura, T.; Murakami, S.; Maeda, T.; Nakamura, T. Liquid−liquid extraction of divalent manganese, cobalt, copper, zinc and cadmium from aqueous chloride solutions by tricaprylmethylammonium chloride. Hydrometallurgy 1984, 12, 245−254. (19) Galan, B.; Urtiaga, A. M.; Alonso, A. I.; Irabien, J. A.; Ortiz, M. I. Extraction of anions with Aliquat 336: Chemical equilibrium modelling. Ind. Eng. Chem. Res. 1994, 33, 1765−1770. (20) Wassink, B.; Dreisinger, D.; Howard, J. Solvent extraction separation of zinc and cadmium from nickel and cobalt using Aliquat 336, a strong base anion exchanger, in the chloride and thiocyanate forms. Hydrometallurgy 2000, 57, 235−252. (21) Juang, R. S.; Kao, H. C; Wu, W. H. Analysis of liquid membrane extraction of binary Zn(II) and Cd(II) from chloride media with Aliquat 336 based on thermodynamic equilibrium models. J. Membr. Sci. 2004, 228, 169−177. (22) Giridhar, P.; Venkatesan, K. A.; Srinivasan, T. G.; Vasudeva Rao, P. R. Extraction of fission palladium by Aliquat 336 and electrochemical studies on direct recovery from ionic liquid chase. Hydrometallurgy 2006, 81, 30−39. (23) Wionczyk, B.; Cierpiszewski, R.; Mól, A.; Prochaska, K. Studies on the kinetics and equilibrium of the solvent extraction of chromium(III) from alkaline aqueous solutions of different composition in the system with Aliquat 336. J. Hazard. Mater. 2011, 198, 257− 268. (24) Masana, A.; Valiente, M. Solvent extraction of zinc(II) in several aqueous chloride media by trilaurylammonium chloride in toluene. Anal. Sci. 1988, 4, 63−68. (25) Lin, H. K. Extraction of zinc chloride with dibutyl butylphosphonate. Metallurg. Trans. B 1993, 24, 11−15. (26) Kyuchoukov, G.; Zhivkova, S. Options for separation of copper(II) and zinc(II) from chloride solutions by Kelex100. Solvent Extr. Ion Exch. 2000, 18, 293−305. (27) Cote, G.; Jakubiak, A. Modelling of extraction equilibrium for zinc(II) extraction by a bibenzimidazole type reagent (ACORGA ZNX-50) from chloride solutions. Hydrometallurgy 1996, 43, 265− 276. (28) Jee, J. G.; Lee, S. H. The pressure effect on the activity coefficient of sodium chloride and bromide ions. Bull. Korean Chem. Soc. 1986, 7, 163−166.

3-[1-(hydroxyimino)undecyl]-1-propylpyridinium bromide showed that the molar ratio of zinc to ligand to chloride in the complex molecule was 1:1:2, 1:1:3, or 1:2:2 and the ligand structure and the aqueous phase composition influenced the values of the equilibrium constants of the extraction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +48616653649. Funding

This work was supported by Grant 32-045/13 DS-PB and realized under Polish−Portuguese scientific and technological co-operation for the years 2013−2014, “Selective extractants for the removal of minor metallic elements from chloride spent pickling baths”. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS NMR analyses were carried out at the Institute of Bioorganic Chemistry Polish Academy of Sciences in Poznan.

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REFERENCES

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Quaternary Pyridinium Ketoximes as Zinc Extractants from Chloride

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