Solvent Extraction of Chromium(III) from Sulphate Medium: A Comprehensive Liquid-Liquid Extraction Study Using Cyanex 302

Cyanex 302 (bis (2,4,4-trimethylpentyl) monothio-phosphinic acid; Fig. 1) was provided by Cytec Canada Inc. and used without additional purification. Chromium(III) sulfate basic Cr₂(SO₄)₃·6H₂O [21% Cr, Loba (India)], sodium carbonate (Na₂CO₃) [99.5%, E. Merck (Germany)], sulfuric acid (H₂SO₄) [98%, E. Merck (India)], potassium chromate (K₂CrO₄) [99%, E. Merck (India)], sodium hydroxide (NaOH) [97%, E. Merck (India)], and hydrogen peroxide (H₂O₂) [30%, E. Merck (Germany)] were used as received, without undergoing any purification processes. Additionally, isobutyl methyl ketone, n-heptane, cyclohexane, toluene, benzene, xylene, and carbon tetrachloride were procured from E. Merck (India), with each compound having a purity of 99%. Kerosene was sourced from the local market and distilled at temperatures ranging from 473 to 533 K to obtain the colorless fraction of kerosene.

Precisely weighing 0.093 g of A. R. K₂CrO₄ was dissolved in distilled water to a final volume of 250 mL to yield a standard Cr(VI) solution, with each 1.0 mL containing 0.10 mg of Cr (VI). Consequently, the concentration of Cr(III) in the aqueous solution was determined using a T60 UV–visible spectrophotometer (PG Instruments Limited, UK) by measuring the absorbance at 360 nm [31]. An edge® dedicated pH/ORP meter (HI2002-01, HANNA INSTRUMENTS, USA) was used to measure the pH of the aqueous phase. Buffer solutions with pH values of 4 and 7 were used for calibration. Anhydrous Na₂CO₃ was added to increase the aqueous pH. Moreover, 0.01 mol/L H₂SO₄ was used to reduce the pH of the aqueous solutions.

The preparation of the Cr(III) stock solution involved dissolving 23.80 g of basic Cr₂(SO₄)₃.6H₂O in a 1000 mL volumetric flask containing distilled water after adding 5 mL of 98% H₂SO₄. This process yielded a stock solution with a sulfate concentration of 0.24 mol/L. A measured portion of this stock solution was further diluted with distilled water in another volumetric flask to obtain a test solution with the required Cr(III) concentration. As long as the pH was maintained between 3.5 and 7.0, and the Cr(III) concentration was maintained at approximately 0.10 g/L, no precipitation of hydroxide was observed.

Cyanex 302, chemically known as bis-(2,4,4-trimethylpentyl) monothiophosphinic acid (Fig. 1), has a molecular weight of 306 g and exists as a dimer. The density of the provided Cyanex 302 was 0.936 mol/L (83%). To prepare a 500 mL stock solution of Cyanex 302 with a concentration of 0.10 mol/L for experimental purposes, 163.4 mL of Cyanex 302 was measured into a 500 mL volumetric flask and filled to the mark with redistilled kerosene.

For the extraction process, a 20 mL portion of the aqueous phase, which contained Cr(III) at specific concentrations and pH levels, was mixed with an equal volume of the organic phase, which included the extractant in kerosene. This mixture was placed in a reagent bottle and agitated for 45 min to extract Cr(III) using Cyanex 302 at a temperature of 303 ± 1 K. The mixing was carried out efficiently using a Stuart Flask Shaker (220 V, 50 Hz) set at 300 strokes per minute. The temperature was maintained by placing the flasks in a thermostatic water bath. After reaching equilibrium, the phases were allowed to settle in the water bath before separation.

The separated aqueous solutions were analyzed to determine the equilibrium pH values, followed by the measurement of Cr(III) concentrations. The concentration of Cr(III) in the organic phase at equilibrium was calculated using the difference or mass balance method. Subsequently, the extraction or distribution ratio (D) was determined by dividing the concentration of Cr(III) in the organic phase by that in the aqueous phase at equilibrium, as illustrated below [5], [14], [15]:

In this context, Co represents the concentration of Cr(III) in the organic phase, while Caq, (eq) denotes the concentration of Cr(III) in the aqueous phase at equilibrium, and Caq, (ini) indicates the initial concentration of Cr(III) in the aqueous phase.

In this section, a measured amount of the organic phase underwent repeated equilibration with equal volumes of fresh Cr(III) aqueous solutions, maintained at a pH of 5.0 and a temperature of 303 K. Following each equilibration, the separated aqueous phase was analyzed to determine its Cr(III) content, and the Cr(III) concentration in the organic phase was inferred by difference. This process was repeated until the organic phase reached Cr (III) saturation.

To strip Cr(III) from the organic phase, three mineral acids, namely H₂SO₄, HCl, and HNO₃, were employed as stripping agents. In the stripping experiment, 10 mL of an organic phase containing Cr(III) at a specific concentration, with kerosene as the diluent, was equilibrated for 45 min at 303 K. This was done with equal volumes of H₂SO₄ solutions at concentrations of 0.10 mol/L (1 stage), 1 mol/L (2 stages), 2 mol/L (2 stages), and 3 mol/L (11 stages). Additionally, two other stripping agents, HCl and HNO₃, were employed in a single-stage stripping process at a concentration of 0.1 mol/L. The stripping ratio was determined by dividing the concentration of Cr(III) in the aqueous phase by its concentration in the organic phase at equilibrium.

The progression of a chemical equilibrium reaction is typically influenced by the temperature. An increase in temperature can either promote the formation of product molecules or favor a reverse reaction. In scenarios where product formation is enhanced, the process absorbs heat and is termed endothermic. Conversely, when the reverse reaction is favored, heat is released, and the process is classified as exothermic. To assess the heat involved in a reaction, the van’t Hoff equation is commonly employed, as follows [3], [5]: 

In solvent extraction, because the equilibrium constant K is directly related to the extraction ratio D, the equation above can be reformulated for the current scenario as follows:

By keeping other variables unchanged, if D values are recorded at different absolute temperatures (T), the graph of log D versus (1/T), K⁻¹, should show a linear relationship. The slope is equal to -(ΔH/(2.303RT)). From the slope of this line, ΔH can be easily determined.

Figure 2 shows a plot of the variation of log D with time in minutes. The log D of Cr(III) increased with the phase contact time up to 30 min, after which it remained constant despite further increases in contact time. Consequently, the equilibration time for the present system was 30 min. In subsequent experiments, a 45-minute phase contact time was adopted to achieve equilibration under a range of experimental conditions.

The initial pH (pH(ini)) significantly influenced the extraction efficiency of Cr(III) from the sulfate medium using Cyanex 302. The experimental results demonstrated that the extraction, represented by the distribution ratio (log D), varied with the initial pH of the aqueous phase. Specifically, the extraction efficiency increased as the pH increased, reaching a maximum at pH 5.0. This phenomenon is illustrated in Figure 3. At this optimal pH, Cyanex 302 concentrations of 0.10 mol/L and 0.15 mol/L achieved extraction percentages of 18.20% and 23.30%, respectively. Beyond this pH, the extraction did not improve, indicating that pH 5.0 provided the most favorable conditions for Cr(III) complex formation and subsequent transfer into the organic phase. This behavior underscores the critical role of pH in controlling the speciation and extractability of chromium ions in the system.

The graph in Fig. 4 illustrates the relationship between log D and log ([Cr(III)](ini), mol/L). It is observed that as the initial concentration of Cr(III) in the aqueous phase increases, the log D value steadily declines. This change was more noticeable at higher Cr(III) concentrations than at lower ones. This finding deviates from the typical principles of solvent extraction. At a Cr(III) concentration of 1.0 g/L, the extraction percentage was merely 8.50% for a Cyanex 302 concentration of 1.0 mol/L and 11.30% for 1.50 mol/L.

The extractant functions as the main reactant, interacting with the metal ion to produce extractable, non-ionic species. It has been consistently observed that increasing the extractant concentration in a system results in a higher extraction percentage or extraction ratio. However, this relationship requires experimental validation. The graph illustrating the relationship between the extractant dependencies and distribution ratio should display a straight line, with the slope corresponding to the number of extractant molecules associated with a metal ion.

Figure 5 illustrates a graph of log D against log ([Cyanex 302](ini), mol/L) at pH 4.0 and 5.0. The results show two distinct straight lines with slopes of approximately 1. This indicates that the dependence of the extractant in the system is one, meaning that a single dimeric molecule of Cyanex 302 is necessary for each Cr(III) to form the extractable species. The unity in extractant dependence implies that at least mono-hydroxo chromium(III) species are extracted in this system [3], [5], [9], [14].

Figure 6 illustrates the relationship between log D and log ([SO₄²⁻], mol/L). The study was performed at two pH values: 4.0 and 5.0. It is evident that sulfate ions negatively affect the extraction of Cr(III) using Cyanex 302. At a sulfate concentration of 0.18 mol/L, the extraction rates of Cr(III) were 21.90% at pH 4.0 and 24.00% at pH 5.0. Alternatively, at a sulfate concentration of 1.50 mol/L, the extraction rates of Cr(III) are 4.90% at pH 4.0 and 7.05% at pH 5.0. The results revealed that Cr(H₂O)₃SO₄OH or Cr₂(H₂O)₄(OH)₂(SO₄)₂ was formed in the aqueous phase and was not extractable by Cyanex 302 [1], [5], [7], [9]. This is likely because the monomeric or dimeric anions of Cyanex 302 cannot replace SO₄²⁻ ions in these aqueous complex species.

Figure 7 presents a graph of log D against (1/T)×10³, K⁻¹. This study included two experimental setups. In both cases, the D values exhibited a pattern similar that to of the temperature changes. The slope and ΔH values are presented in the figure. For the Cyanex 302 system, ΔH values of 25.427 kJ/mol and 19.473 kJ/mol were recorded for concentrations of 0.10 mol/L and 0.15 mol/L, respectively. A positive ΔH indicates that the process is endothermic.

The percentage of Cr (III) extraction by Cyanex 302 using various diluents is depicted in Figure 8. The figure shows that toluene achieved the highest extraction percentage of 28.28%. In contrast, other diluents such as isobutyl methyl ketone, n-heptane, cyclohexane, benzene, xylene, carbon tetrachloride, and kerosene exhibit extraction percentages of 15.85, 20.02, 17.39, 14.01, 18.89, 25.27, and 17.76, respectively, for Cr(III) by Cyanex 302. In this study, kerosene was chosen as a diluent because of its easy availability.

A 100 mL aliquot of a 0.10 mol/L Cyanex 302 solution in kerosene was repeatedly equilibrated with 100 mL portions of fresh aqueous solution. These solutions contained 0.35 g/L Cr(III) and 0.75 mol/L SO₄²⁻, and the process was conducted at pH 5.0. The temperature was consistently maintained at 303 K throughout the experiment. The organic phase reached its maximum Cr(III) loading by the 10th contact (Fig. 9). In this study, the highest loading achieved was 0.839 g/L Cr(III). The loading capacity, which refers to the maximum amount of a metal ion, in grams, that can be extracted from 100 g of extractant, is an essential factor when choosing an extractant for a metal-ion extraction system, particularly in industrial applications. Calculations revealed that when Cyanex 302 is dissolved in kerosene, its loading capacity for Cr(III) is 2.742 g of Cr(III) per 100 g of Cyanex 302.

The stripping experiment used H₂SO₄, HCl, and HNO₃ as stripping agents. Table 1 presents the results of this study in detail. In a single stage, sulfuric acid at a concentration of 0.10 mol/L can strip 2.12%, while concentrations of 1.0 and 2.0 mol/L can achieve stripping of 9.31% and 15.28% in two stages, respectively. Furthermore, a 3.0 mol/L concentration of sulfuric acid achieved a 28.68% strip in 11 stages. Alternatively, at a concentration of 0.10 mol/L, HCl and HNO₃ acids can strip 1.38% and 0.37% in a single stage, respectively. It has been observed that HCl and HNO₃ acids are unsuitable as stripping agents in the current investigation.

Table 2 shows how Cr(III) is extracted using different solvents. It highlights the key factors in choosing a solvent for Cr(III), a metal that is difficult to extract from water. Cationic extractants like D2EHPA and Cyanex 272 work best. They use an acid-exchange reaction, which is very sensitive to pH. The process can be over 99% efficient, but only if the pH is between 3.5 and 5.5. Outside this range, efficiency drops significantly because of either insufficient deprotonation at low pH or Cr(III) breakdown and the formation of solids at high pH.

In this study, Cyanex 302, an acidic extractant, only extracted 23% at pH 5. Chelating Extractants like LIX 84-I are better because they can separate Cr(III) from Fe(III) at about pH 6, whereas D2EHPA is less effective. However, they work very slowly, taking hours or even days, which is a big problem for cost and operation. Tri-n-butyl phosphate (TBP) does not extract Cr(III) well unless used with high levels of certain anions to form extractable neutral complexes. Even under tough conditions, it extracted only 28%, indicating it is not suitable for Cr(III).