Hexavalent chromium recovery by liquid–liquid extraction with 2-octylaminopyridine from acidic chloride media and its sequential separation from other heavy toxic metal ions

2.1 Apparatus

An Elico-digital spectrophotometer model SL-171 (Elico India Limited, Hyderabad) with 1 cm quartz cells was used for absorbance measurement. An Elico digital pH meter model LI-120 (Elico India Limited, Hyderabad) was used for pH measurements. Tapson’s analytical single pan balance model 200 T having accuracy 0.001 g was used for weighing operations.

2.2 Reagents

2.3 General extraction procedure for the extraction of chromium(VI)

To an aliquot containing 25 μg chromium(VI) in 25 mL volumetric flask enough hydrochloric acid was added to get the concentration of the solution 0.5 M. The solution was diluted to 25 mL up to the mark and transferred into a 125 mL separatory funnel. The aqueous phase was equilibrated with 10 mL of 0.036 M 2-OAP solution in xylene for 5 min. The two phases were allowed to separate. The aqueous phase was washed with 5 mL xylene and was rejected. The organic phase was back-stripped with three 10 mL portions of 7 N ammonia. The solution was evaporated till it remains 1/4th. The chromium(VI) from the solution was determined by spectrophotometric method using diphenyl carbazide (Cotton and Wilkinson, 1972; Greenwood and Earnshaw, 1984) at 540 nm. The amount of chromium(VI) was computed from the calibration curve.

3.1 Effect of acidity

The extraction of 25 μg of chromium(VI) from different concentrations of acid media with 0.036 M 2-OAP in xylene was carried out by keeping the aqueous to organic volume ratio 2.5:1. The extraction of chromium(VI) was 14.3% at 0.01 M hydrochloric acid and becomes quantitative in the range of 0.4–0.8 M hydrochloric acid and again there was decrease in the extraction of chromium(VI) with increasing the acidity. It may be due to the formation of stable hydrochloride with 2-OAP. The percentage extraction (% E) and distribution ratio (D) are calculated by using following expressions:

3.2 Extraction of chromium(VI) as a function of 2-OAP concentration

The extraction of 25 μg of chromium(VI) from 0.5 M hydrochloric acid solution by 2-OAP in xylene within the range 0.0048–0.4850 M 2-OAP was studied at 25 °C. The extraction of chromium(VI) was found to increase with increasing reagent concentration. The extraction was quantitative in the concentration range of 0.024–0.073 M of 2-OAP. Further increase in concentration of 2-OAP decreased the extraction of chromium(VI). The decrease in the extraction is due to the formation of stable ion-pair complex between 2-OAP and chloride. However, in this procedure in order to ensure complete extraction of chromium(VI), 0.036 M 2-OAP concentration was recommended.

3.3 Extraction of chromium(VI) as a function of diluents

The extraction of chromium(VI) was performed from 0.5 M hydrochloric acid medium using 10 mL of 0.036 M 2-OAP in various aliphatic and aromatic diluents, like benzene, toluene, xylene, chloroform, carbon tetrachloride, 1,2-dichloroethane, methyl isobutyl ketone (MIBK), n-butanol, amyl acetate, amyl alcohol and kerosene, Table 2. The extraction of chromium(VI) was found to be quantitative in xylene, while in other diluents there was incomplete extraction of chromium(VI). No correlation between dielectric constant and percentage extraction was observed.

3.4 The influence of equilibrium time

The shaking time was varied from 0.25 to 20 min. It was observed that the extraction of chromium(VI) was 78.8% at 0.25 min, which was further increased and became quantitative in the range of 4–7 min. With prolonged shaking time the percentage extraction decreased due to dissociation of the ion-pair complex due to agitation. For proposed method 5 min equilibration time was recommended in order to ensure the complete extraction of metal ion.

3.5 Loading capacity of 2-OAP

The loading capacity of the extractant was determined by the repeated contact of organic phase with a fresh feed solution of the metal of same concentration. For a 10 mL, 0.036 M solution of 2-OAP in xylene at 0.5 M hydrochloric acid concentration and a A/O of 2.5:1, the maximum loading capacity for chromium(VI) was found to be 75 μg at 25 °C.

3.6 Effect of aqueous to organic volume ratio

The results of contacting different volumes ratios of aqueous to organic phase have been investigated. The study indicated that a preferred aqueous to organic (A/O) phase ratio in the study was 7.5:1 or less. This is evident from the sharp increase in the extraction efficiency as well as distribution ratio of chromium(VI) when phase ratio was changed from 25:1 to 7.5:1.This may be simply due to the unavailability of the reagent for metal extraction. So a crowding effect occurs at low phase ratio. However, in the recommended procedure the phase ratio was mentioned as 2.5:1 so as to avoid the large consumption of the hydrochloric acid.

3.7 Effect of stripping agents

Various stripping agents of different concentrations were examined for the back extraction of the chromium(VI) from the organic phase, Table 3. Chromium(VI) was stripped quantitatively by ammonia solution (3 × 10 mL) in the concentration range of 7–9 N while other basic strippants like sodium hydroxide, potassium hydroxide and sodium carbonate and weak acid like acetic acid were found to be ineffective in the stripping of chromium(VI). In the proposed method 7 N (3 × 10 mL) ammonia was used as a strippant for further study.

3.8 Nature of the extracted species

Attempts were made to ascertain the stoichiometry of extracted complex species using slope analysis method, using log D_[Cr(VI)] against log C_[2-OAP] plots. A graph of log D_[Cr(VI)] against log C_[2-OAP] at 0.1 and 0.3 M hydrochloric acid concentrations gave slopes of 1.0 and 0.9 respectively, Fig. 1. The probable mechanism of the extraction appears to be protonation of 2-OAP which forms cationic species like [2-OAPH⁺], while the chloride ions combine with chromium(VI) to form anionic species like [CrO₃C1⁻] (Marckzenko, 1976; Sandell, 1965). The probable nature of the extracted species is [(2-OAPH⁺·CrO₃C1⁻]_(org).

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Figure 1

Log–log plot of log D_[Cr(VI)] versus log C_[2-OAP] at 0.1 and 0.3 M HCl.

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The extraction mechanism was explained as:

3.9 Effect of various diverse ions on the extraction of chromium(VI)

In order to judge the selectivity and utility of the proposed method, chromium(VI) was extracted in the presence of several other ions. The tolerance limit was set at the amount of the foreign ion that could be present to give an error less than ±2% in the recovery of chromium(VI). It has been found that I⁻, $\text{NO}_2{}^{-}$ , SCN⁻, ${\text{S}}_2\text{O}_3{}^{2-}$ , thiourea, ascorbate, Te(IV), Fe(III), Cu(II), T1(I) ions were interfered in the extraction of chromium(VI). The interference of Cd(II) and Bi(III) was eliminated by masking them with 25 mg EDTA while interference of Tl(III) and Sb(III) was eliminated by masking with 5 mg fluoride, Table 4.

4.1 Separation and determination of chromium(VI) from binary mixtures

The suitability of the above developed method was examined by applying it to the separation and determination of chromium(VI) in a variety of binary mixtures containing associated and toxic metal ions, e.g. Cr(III), Ni(II), Co(II), Pb(II), Zn(II), Hg(II), Mn(II) and Mn(VII). The extraction and determination of chromium(VI) from binary mixtures were carried out by taking the advantage of difference in the extraction conditions of the metal ions, Table 5.

Chromium(VI) was separated from the binary mixture of Ni(II), Mn(II), Mn(VII) and Co(II) by its extraction with 10 mL of 0.036 M 2-OAP in xylene from 0.5 M hydrochloric acid solution. Under these conditions the added metal ions remained quantitatively in the aqueous phase, while chromium(VI) was extracted in the organic phase. The aqueous phase was washed with 5 mL xylene to remove traces of the reagent. It was evaporated to moist dryness with 10 mL concentrated hydrochloric acid. The residue was dissolved in distilled water and diluted to 25 mL. The metal ion from aqueous phase was determined by the standard methods (Cotton and Wilkinson, 1972; Greenwood and Earnshaw, 1984; Marckzenko, 1976). Chromium(VI) from organic phase was back extracted with 7 N ammonia solution (3 × 10 mL) and estimated by spectrophotometric method.

The chromium(VI) was separated from the binary mixture of Cr(III), Pb(II), Zn(II) and Hg(II) by its extraction with 10 mL of 0.036 M 2-OAP in xylene from 0.5 M hydrochloric acid solution. Under these conditions the added metal ions were co-extracted with chromium(VI) in trace amount in organic phase. Separation of these metal ions was achieved by the use of masking agents. Chromium(III), Zn(II) and Hg(II) were masked with 25 mg EDTA while Pb(II) was masked by 10 mg EDTA. Under these conditions the added metal ions remained in the aqueous phase quantitatively while the chromium(VI) was extracted in organic phase. The aqueous phase was washed with 5 mL xylene. It was evaporated to moist dryness with 2 mL perchloric acid. The procedure was repeated thrice for complete demasking. The residue was evaporated with 5 mL hydrochloric acid and then with 10 mL water twice for the removal of trace amount of acid. The residue was dissolved in water, diluted to 25 mL and the metal ions from the aqueous phase were determined by standard methods (Cotton and Wilkinson, 1972; Greenwood and Earnshaw, 1984; Flaschka and Barnard, 1972). Chromium(VI) from the organic phase was back extracted with 7 N ammonia (3 × 10 mL). The aqueous phase was evaporated to one fourth of its total volume and chromium(VI) was determined by diphenyl carbazide method.

4.2 Sequential separation and determination of chromium(VI) from ternary mixtures containing toxic metals

Sequential separation of chromium(VI) from ternary mixtures of Cr(VI), Pb(II), Zn(II) and Cr(VI), Pb(II) and Hg(II) was carried out by taking advantage of extraction ability of 2-OAP with metal ions in mineral acid medium and weak organic acid medium, Table 6.

Under the optimum extraction condition of chromium(VI), Pb(II) and Zn(II) were masked with 25 mg EDTA. The masked metal ions remained in the aqueous phase, while chromium(VI) was quantitatively extracted in the organic phase which was back extracted with 7 N ammonia (3 × 10 mL) and was determined spectrophotometrically. Lead(II) and Zn(II) remained in the aqueous phase were demasked by perchloric acid, followed by hydrochloric acid treatment. The residue was dissolved in 10 mL water and evaporated to moist dryness to remove trace amount of acid. Lead(II) was separated from Zn(II) from its extraction with 10 mL of 0.036 M 2-OAP in chloroform from 0.007 M sodium succinate solution at pH 10 (Mane and Anuse, 2008). Under these conditions Zn(II) was remained quantitatively in the aqueous phase. The aqueous phase was washed with 5 mL chloroform to remove traces of the reagent. Zinc(II) from the aqueous phase was determined by standard method (Flaschka and Barnard, 1972). Lead(II) from the organic phase was backstripped with 0.4 M hydrochloric acid (3 × 10 mL) and estimated by standard method (Flaschka and Barnard, 1972).

Under the optimum extraction condition of chromium(VI), Pb(II) and Hg(II) were masked with 25 mg EDTA, while chromium(VI) was quantitatively extracted in organic phase, it was back extracted with 7 N ammonia (3 × 10 mL) and was determined by recommended procedure. Lead(II) and Hg(II) remained in the aqueous phase were demasked by perchloric acid followed by hydrochloric acid treatment. The residue was dissolved in 10 mL distilled water and evaporated to moist dryness to remove traces of acid completely. The residue containing Pb(II) and Hg(II) was dissolved in distilled water. Under the optimum extraction condition of Pb(II) there was partially co-extraction of Hg(II), but quantitative co-extraction of Hg(I). Hence Hg(II) was reduced to Hg(I) with hydroxylamine hydrochloride. Lead(II) and Hg(I) were extracted with 10 mL of 0.036 M 2-OAP in chloroform from 0.007 M sodium succinate solution at pH 10. Lead(II) was back stripped from the organic phase with 0.4 M acetic acid (3 × 10 mL) and determined by standard method, (Flaschka and Barnard, 1972) while Hg(I) remained quantitatively in organic phase was converted into Hg(II) by warming the organic phase with bromine water. Mercury(II) was back extracted with 2 N HCl (3 × 10 mL) and determined by standard method (Flaschka and Barnard, 1972).

4.3 Validity of method

To study applicability of the proposed method for the real samples, the method was employed for the analysis of chromium(VI) present in real samples like alloys, Table 7 and waste water samples from tannery industries, Table 8.