Liquid-liquid extraction equilibrium study of vanadium(V) from nitrate medium by technical grade D2EHPA dissolved in kerosene

Technical grade D2EHPA was acquired from Fluka, Switzerland. Figure 2 displays the chemical structure of pure D2EHPA. Ammonium metal vanadate (NH₄VO₃) (99%, Riedel-deHain, Germany) was used as a source of vanadium (V). Kerosene was purchased from the local market. Collected kerosene was redistilled over 473-533 K. The redistilled kerosene was colorless and aliphatic. All the other chemicals were of reagent grade and used without further purification.

The aqueous phase vanadium (V) content was estimated spectrophotometrically at 450 nm using T60 Visible Spectrophotometer (PG Instruments Limited, United Kingdom). The H₂O₂-H₂SO₄ method was used [32], [33]. Accurately weighed 2.296 g of A. R. ammonium meta-vanadate, on dissolving with water and making up to 1.0 L, gave a standard vanadium (V) solution, in which 1.0 mL contained 1.0 mg vanadium (V). The aqueous phase pH was measured by the edge® dedicated pH/ORP meter (HI2002-01, HANNA INSTRUMENTS, USA). Buffer solution of pH 4 and 7 was used to standardize the pH meter. Moreover, the aqueous phase pH for the experimental purpose was adjusted by anhydrous Na₂CO₃.

The requisite amount of NH₄VO₃ was dissolved in 535 mL of 15.44 mol/L HNO₃ solution and diluted to 2000 mL to obtain a stock solution containing 10.75 g/L vanadium (V) and 4.14 mol/L HNO₃. An aliquot of 25 mL of technical grade D2EHPA was dissolved in redistilled kerosene in a 500 mL volumetric flask and made up to the mark to get a 500 mL stock solution of 5% D2EHPA [34], [35].

An aliquot of the aqueous phase (20 mL) containing vanadium (V) and nitrate ion at definite concentrations and having definite pH was agitated with an equal aliquot of the organic phase containing the extractant in kerosene (taken in a reagent bottle) for a predetermined time (30 min) at temperature 303±1 K. The phase agitation was made effective by a Stuart Flask Shaker (220 V, 50 Hz, UK). The temperature was controlled by putting the agitation flasks in a thermostatic water bath (LAUDA B 4 Stainless Steel Bath, Thomas Scientific, USA). After equilibration, phases were allowed to settle within the water bath and disengaged.

Separated aqueous solutions were subjected to determine equilibrium pH values followed by vanadium (V) concentrations. The vanadium (V) concentration in the organic phase at equilibrium was calculated by difference or mass balance. The extraction or distribution ratio (D) was calculated by dividing the concentration of vanadium (V) in the organic phase by that in the aqueous phase at equilibrium. The extraction ratio, D, was then calculated using the following relation:

In loading, an aliquot of 100 mL 5% (v/v) technical grade D2EHPA solution dissolved in kerosene was repeatedly equilibrated with 100 mL portions of the fresh aqueous solution containing 1.66 g/L vanadium (V) and 0.20 mol/L nitrate at pH of 2.00 and temperature 303 K. After each equilibration, the separated aqueous phase was analyzed for its vanadium (V) content. The organic phase vanadium (V) concentration was calculated by the difference. The process continued until the organic phase was saturated with vanadium (V).

The stripping procedure was performed by taking 10 mL of an organic phase containing 3.24 g/L vanadium (V) was equilibrated for 30 min at 303 K with an equal volume of either 0.10 mol/L NH₄OH, 0.50 mol/L NH₄OH, 0.10 mol/L NH₄OH + 0.10 mol/L NH₄Cl, 0.90 mol/L NaOH, 0.10 mol/L H₂SO₄, 0.50 mol/L H₂SO₄, 1.0 mol/L H₂SO₄, or 0.50 mol/L HNO₃ acid solutions. The stripping ratio was calculated by dividing vanadium (V) concentration in the aqueous phase by that in the organic phase at equilibrium [36].

The variation of log D with the variation of the contact time (min) is depicted in Fig. 3. It is found that the concentration ratio of vanadium (V) increases with the increase in phase contact time up to 30 min. With the further increase in the contact time, the value of log D remains unchanged. Therefore, it is concluded that the equilibration time for the current system is about 30 min. In subsequent experiments, a phase contact time of 30 min has been used.

 Metal-ion may be polymerized in both aqueous and organic phases, and co-existing anion complex may be formed with increasing metal-ion and ligand concentrations. In such cases, it is expected that the extraction ratio should vary with the metal ion concentration in the aqueous phase. The log D vs. log([V(V)(ini)], mol/L) plots are shown in Fig. 4. It is found that in both cases under investigation, the extraction ratio decreases with the increase in the initial concentration of vanadium (V) in the aqueous phase. The slope of log D vs log([V(V)(ini), mol/L) plots are steeper at higher concentration regions than those at lower concentration region. Therefore, the system appears as non-ideal. The non-ideality, i.e., the decrease in extraction ratio with the increase of vanadium (V) concentration, may be due to the following reasons:

i) Non-constancy of aqueous phase acidity, pH

The equilibrium pH may drop with the increase in the initial vanadium (V) ion concentration in the aqueous phase. This phenomenon is caused by the liberation of H⁺ during the extraction process (Eq. 2). It can also rise due to the consumption of H⁺ from the solution to form extractable species (Eq. 3).

In the present case, the dropping down of pH_(eq) from pH_(ini) (Fig. 4) is noticed, and this indicates the extraction of the metal ion by chelation mechanism [27], [37].

ii) Non-constancy of equilibrium extractant concentration

With the increase in the initial vanadium (V) concentration, more extractants will be consumed to form the extractable complex. As a result, the equilibrium extractant concentration will be decreased with the increase in the initial vanadium (V) concentration in the aqueous phase, and the extraction ratio or percent extraction will be decreased, as indicated by the following equation.

where, D = extraction ratio, K_ex = extraction equilibrium constant, pH = equilibrium pH of the aqueous phase, [H₂A₂]_(ini) = initial extractant concentration in the organic phase, S = concentration of extractant which is used in forming the metal complex.

Besides these two causes, the decrease in extraction ratio with increasing the initial vanadium (V) concentration may be attributed to the non-ideality of the aqueous phase, i.e., hydrolysis and polymerization of vanadium (V) species in the aqueous phase.

It is known that VO₂⁺ forms V₁₀O₂₆(OH)₂^4- on lowering acidity (increasing pH), and this conversion process is probably facilitated by the increase in VO₂⁺ concentration in the aqueous phase. This conversion might be of the following type:

 Invariably, all extraction processes involving either ion-pair formation or ion exchange mechanism depend on the acid concentration or pH of the medium from which the metal ion is extracted [26], [38]. In general, for an ion-pair extraction, the extraction ratio is increased with increasing the concentration of acid (i.e., with decreasing the pH), whereas for liquid cation exchange and chelation system, the extraction ratio is decreased with increasing the concentration of acid (i.e., with decreasing the pH). The extractant, D2EHPA (and the co-existing M2EHPA), is a liquid cation exchanger [19], [39], [40]. When such an extractant extracts a metal ion, a reversible ion-exchange reaction occurs in which the metal ion exchanges the hydrogen ion of the extractant. For a cation exchange system, the slope of log D versus pH or –log [H⁺] plots represent the number of hydrogen ions (H⁺) liberated for each metal-ion from the extractant during extraction. So, the extraction equilibrium will depend on the pH, in the aqueous phase, and the extractant concentration in the organic phase.

The plot of log D vs. pH_(eq) is depicted in Fig. 5. Figure 5 shows that the extraction of vanadium (V) increases with the aqueous equilibrium pH up to ~2.50. This phenomenon is the typical characteristic of ion-exchange or chelation type of extraction. However, vanadium (V) extraction is decreased with the increase of aqueous pH above pH_(eq) = 2.50, which indicates the formation of solvated ion pair. The limiting slopes of the two portions of the curve are calculated. For cation exchange reactions, the pH dependency (slope of log D vs. pH_(eq) plot) should be positive due to the liberation of H⁺ by extraction reaction. This slope is 0.35. The negative pH-dependency (negative slope of log D vs. pH_(eq) plot) is indicative of the association of H⁺ with existing vanadium (V) to form the extractable species, and this slope is estimated as -0.32.

According to Zeng and Cheng [41], yellow colored VO₂⁺ exists within pH 1-2, whereas, within pH 2-6.50, orange red V₁₀O₂₈^-6 exists. However, it is also reported that with the gradual increase of pH from zero, VO₂⁺ is gradually transformed to VO(OH)₃ (to a small extent), V₁₀O₂₆(OH)₂^4-, V₁₀O₂₇(OH)^5-, V₁₀O₂₈^6-, V₃O₉^3-, V₄O₁₂^4-, VO₂(OH)₂^- etc. V₁₀O₂₆(OH)₂^4- is virtually doubly protonated V₁₀O₂₈^6-; whereas V₁₀O₂₇OH^5- is single protonated V₁₀O₂₈^6-. The distribution of these species as a function of pH is also available, as shown in Table 1.

Figure 5 also includes the % of existing vanadium (V) species at various pH values. It is seen that aqueous pH of value up to 1.50 in solution persists to exist VO₂⁺ mainly (98%). Hence, the principal reaction responsible for V(V) extraction in this region follows the ion-exchange reaction.

 According to the above reaction, the pH dependence should be 1; but the experimental value is only 0.35. Therefore, it is considered that a parallel reaction product formation is also responsible for V(V) extraction. It will be seen later that [NO₃^-] enhances extraction. Moreover, it is thought that the ion-pair VO₂NO₃ is solvated by H₂A₂ and transferred to the organic phase. The probable equation for solvated ion-pair formation can be depicted as follows:

The reaction in Eq. (7) is independent of [H⁺]. Therefore, it is concluded that in the higher acidity region, 35% portion of V(V) is extracted following the reaction presented in Eq. (6) and the rest by the reaction shown in Eq. (7). Hence, the overall pH dependence becomes 0.35.

On increasing the pH of the aqueous solution, VO₂⁺ concentration in the system drops abruptly and becomes negligible at pH_(eq) 3. Consequently, the concentration of the hydrolysed product V₁₀O₂₆(OH)₂^4- increases proportionally and exists in the highest proportion at pH_(eq) 3. The prominent extractable aqueous V(V) species is V₁₀O₂₆(OH)₂^4-. The following type of reaction probably extracts this species:

Moreover, the formation of V₁₀O₂₇(OH)^5- starts at pH ~3 and becomes maximum at around pH 5. The extraction of this species is ruled out because the formation of H₆V₁₀O₂₈.nH₂A₂ will require 5 H⁺, which is less likely supplied by the aqueous solution of high pH. In Eq. (8), 4 H⁺ are needed for 10 vanadium (V), so the pH dependence would be -0.40. The experimental value is -0.32, so the explanation given above conforms to the experimental results.

The investigated system's pH functionality is exceptionally low, which may be a drawback.

The effect of the extraction ratio of a metal ion on extractant concentration is essential in the solvent extraction study. In all cases, the extraction ratio increases with increasing the extractant concentration. This phenomenon is the expected behavior of the extractant to extract the metal ion. Therefore, the study on the effect of extractant concentration on extraction ratio is valuable in obtaining an extractant's selectivity towards a metal ion. It also helps to explain the extraction mechanism to better understand a particular extraction system. The extractant, D2EHPA, and admixed M2EHPA are liquid cation exchangers. These extractants can form chelate complexes with metal ions. It can extract metal ions in both monomeric and dimeric forms. In almost all extraction reactions, the extraction ratio increases with the extractant concentration increases in the organic phase. The relation log D = log K_ex+ z log [D2EHPA]_(o)-y log [H⁺] indicates that the slope of log D versus log [D2EHPA] plots should indicate the number of moles of D2EHPA used up per mole of vanadium (V) in forming the extractable species. So, it is the most crucial parameter to investigate the type of extraction reaction that occurs during extraction.

Figure 6 represents the plot of log D vs. log ([D2EHPA], %) for two different metal ion concentrations. In both cases, straight lines are obtained. The slopes of these straight lines are 0.234 and 0.239 for 0.56 g/L and 0.72 g/L vanadium (V) systems, respectively. The results indicate that the more the extractant concentration, the more V(V) extraction. This behavior is normal for any extraction process. The slope values indicate the requirement of one extractant molecule per four vanadium (V) extracted. As the equilibrium pH varies with the variation of extractant concentration, it is necessary to get log D versus log ([D2EHPA], %) plot at constant equilibrium pH values. The positive extractant dependence supports the general phenomenon of an extraction process.

Co-existing anion in the aqueous phase often affects the extraction characteristics of a metal ion by an extractant. In a few cases, it is found that the extraction is independent of co-existing anion concentration. Since the extraction has been carried out from the nitrate medium in the present study, it is necessary to determine the effect of nitrate ion concentration on the extraction characteristics of the current system.

The log D vs. log ([NO₃^-], mol/L) plot for two different D2EHPA concentration systems is shown in Fig. 7. A curve is obtained instead of a straight line in each case. The extent of variation of extraction ratio with the [NO₃^-] depends on its concentration region. In both cases, the tangential positive slopes of these curves are increased gradually with the increase in the nitrate ion concentration. The slope approaches zero at lower nitrate ion concentration regions, whereas in the high concentration regions, it gradually increases to about 0.69 at [NO₃^-] of 1.50 mol/L. The nitrate ion thus exhibits the salting-out effect in the present system.

The effect of temperature in the present system has been determined at two different concentrations (5% and 7%). The experimentation related van’t Hoff [42] (log D vs. (1/T), K^-1) plots are given in Fig. 8. In both cases; it is found that the extraction ratio is decreased with increasing temperature, and the straight-line relationship holds in the van’t Hoff plots. Slopes of the line are 655.2 and 619.4 for 5% and 7% D2EHPA systems, respectively. From the slopes of the plots, the values of the heat of extraction reaction (ΔH) have been calculated as -12.54 kJ/mol and -12.14 kJ/mol for 5% and 7% D2EHPA systems, respectively. The extraction process is, therefore, exothermic with ΔH value of about -12.34 kJ/mol.

An aliquot of 100 mL 5% technical grade D2EHPA solution in kerosene has been repeatedly equilibrated with 100 mL of fresh aqueous solution at a pH of 2.00 and a temperature of 303 K. The fresh aqueous solution contains 1.66 g/L vanadium (V) and 0.20 mol/L NO₃^-. The cumulative vanadium (V) concentration in the organic phase has been plotted against the phase contact number in Fig. 9.

It is observed that the organic phase loading with vanadium (V) is almost ended at the 4th contact. It is seen from the Fig. 9 that the saturation of 5% or 53 g/L D2EHPA solution with vanadium (V) occurs at the 4th contact when the extractant is repeatedly contacted with 1.66 g/L vanadium (V) solution at pH_(ini) of 2. The cumulative [vanadium (V)] increases gradually with the contact number and reaches the value of ~3.24 g/L in the organic phase. This result indicates that 53 g technical grade extractant can extract 3.24 g vanadium (V). It suggests a loading capacity of 6.11 g vanadium (V) per 100 g extractant. The loading capacity is low for its industrial implication.

This study has been carried out as the diluent may tremendously affect the metal ion distribution in a solvent extraction process. For this, the extraction ratios have been measured when the same aqueous phase has been extracted by 5% technical grade D2EHPA solutions dissolved in different diluents keeping all other conditions identical. The results are presented in Table 2.

It is observed that the extraction ratio increases in the following order with the variation of diluent: chloroform (CHCl₃) (D = 1.02) < n-heptane (C₇H₁₆) (D = 1.11) < cyclohexane (cyclo-C₆H₁₂) (D = 1.12) < carbon tetrachloride (CCl₄) (D = 1.22) < isobutyl methyl ketone (C₆H₁₂O) (D = 1.29) < toluene (C₇H₈) (D = 1.31) < benzene (C₆H₆) (D = 1.57) < 1, 2-dichloroethane (C₂H₄Cl₂) (D = 1.58) <chlorobenzene (C₆H₅Cl) (D = 1.73) < xylene (C₈H₁₀) (D = 2.81) < kerosene (D = 4.89). The study helps draw the conclusion that kerosene, xylene, and chlorobenzene are suitable diluents for the extraction of vanadium (V) by technical grade D2EHPA. Due to availability, less volatility, low price, and high extraction power for this extraction system, kerosene is preferable.

The stripping process is studied with 0.10 mol/L NH₄OH, 0.50 mol/L NH₄OH, 0.10 mol/L NH₄OH + 0.10 mol/L NH₄Cl, 0.90 mol/L NaOH, 0.10 mol/L H₂SO₄, 0.50 mol/L H₂SO₄, 1.0 mol/L H₂SO₄, 0.50 mol/L HNO₃ solution at temperature of 303 K and O/A of 1. The stripping results are given in Table 3.

The stripping percentage is much better with 0.50 mol/L H₂SO₄ than other stripping agents. Stage-wise stripping has been investigated with an H₂SO₄ solution of 0.50 mol/L concentration. About 91.35% can be stripped from the loaded organic phase with the repeated five contacts with the new 0.50 mol/L H₂SO₄ solution.