Extraction of chromium(VI) by salting-out with a homogeneous, mixed solvent of water and 2-propanol: A laboratory study

Abstract

Background, Aims and Scope

Chromium enters into the aquatic environment as a result of effluent discharge from steel works, electroplating, leather tanning industries and chemical industries. As the Cr(VI) is very harmful to living organisms, it should be quickly removed from the environment when it happens to be contaminated. Therefore, the aim of this laboratory research was to develop a rapid, simple and adaptable solvent extraction system to quantitatively remove Cr(VI) from polluted waters.

Methods

Aqueous salt-solutions containing Cr(VI) as CrO ²⁻₄ at ppm level (4–6 ppm) were prepared. Equal volumes (5 ml) of aqueous and organic (2-PrOH) phases were mixed in a 10 ml centrifuge tube for 15 min, centrifuged and separated. Concentrations of Cr(VI), in both the aqueous and organic phases, were determined by atomic absorption spectrometry. The effects of salt and acid concentrations, and phase-contact time on the extraction of Cr(VI) were investigated. In addition, the extraction of Cr(VI) was assessed in the presence of tetramethylammonium chloride (TMAC) in 2-PrOH phase. Effects of some other metals, (Cd(II), Co(II), Cu(II), Ni(II) and Zn(II)), on the extraction of Cr(VI) were also investigated.

Results and Discussion

The Cr(VI) at ppm level was extracted quantitatively by salting-out the homogeneous system of water and 2-propanol(2-PrOH) using chloride salts, namely CaCl₂ or NaCl, under acidic chloride media. The extracted chemical species of Cr(VI) was confirmed to be the CrO₃Cl⁻. The ion-pair complex extracted into the organic phase was rationalized as the solvated ion-pair complex of [2-PrOH ⁺₂ , CrO₃Cl⁻]. The complex was no longer stable. It implied the reaction between extracted species. Studies revealed that salts and acid directly participated in the formation of the above complex. Use of extracting agents (TMAC) didn’t show any significant effect on the extraction of Cr(VI) under high salting-out conditions. There is no significant interference effect on the extraction of Cr(VI) by the presence of other metals. The Cr(VI) in the organic phase was back-extracted using an aqueous ammonia solution (1.6 mol dm⁻³) containing 3 mol dm⁻³ NaCl. The extraction mechanism of Cr(VI) is also discussed.

Conclusions

Salting-out of homogeneous mixed solvent of 2-propanol can be employed to extract Cr(VI) quantitatively, as an ion-pair of [2-PrOH ⁺₂ ⊙ CrO₃Cl⁻] solvated by 2-PrOH molecules. Then, the complex becomes ‘solvent-like’ and is readily separated into the organic phase. The increase of Cl-ion concentration in the aqueous phase favors the extraction. The 2-PrOH, salts and acid play important roles in the extraction process. There is no need to use an extracting agent at a high salting-out condition.

Recommendations and Perspectives

Chromium(VI) must be quickly removed before it enters into the natural cycle. As the 2-PrOH is water-miscible in any proportion, ion-pairing between 2-PrOH ⁺₂ and CrO₃Cl⁻ becomes very fast. As a result, Cr(VI) can easily be extracted. Therefore, the method is recommended as a simple, rapid and adaptable method to quickly separate Cr(VI) from aqueous samples.