Extraction mechanism of Cr(VI) on the aqueous two-phase system of tetrabutylammonium bromide and (NH₄)₂SO₄ mixture

Abstract

An aqueous two-phase system of tetrabutylammonium bromide (TBAB) and (NH₄)₂SO₄ mixture has been developed for the extractive preconcentration and separation of certain compounds. TBAB concentration in upper phase is much higher than that of bottom solution. This new aqueous two-phase system is proposed for the highly selective extraction of Cr(VI) from large amounts of Cr³⁺. The Cr(VI) is found to be extracted into the TBAB-rich upper phase due to ion pair formation such as HCrO₄⁻ · TBAB⁺. The Cr(VI) was sufficiently extracted into the upper phase in the pH range from 1 to 5. The proposed method has been applied to the determination of trace of Cr(VI) in wastewater samples with a coefficient of variation less than 3.2%. The recovery obtained was not lower than 90%. The determination limit for the Cr(VI) was found to be 60 μg l⁻¹ in 10 ml of sample solution.

Introduction

Chromium is widely used in the electro-industry, stainless steel production and chrome plating but Cr(VI) is well known to be important as an environmental contaminant because of its high toxicity and wide dispersion from industrial wastes. Hexavalent chromium compounds are considerably toxic and known to be carcinogenic, while trivalent chromium is virtually non-toxic and considered an essential element in human diet at low concentration levels. According to World Health Organization (WHO) guidelines for drinking water, the permissible level of chromium is 0.05 mg l⁻¹. Many studies have hitherto been made for the selective determination of Cr(III) or Cr(VI). The most widely used methods for the mutual separation of Cr(III) and Cr(VI) in water samples including sea water are: (a) complexation with dithiocarmamate [1], [2], [3] followed by extraction into organic solvents; (b) coprecipitation with inorganic hydroxides, such as those of Fe(III) [4] and Bi(III) [5] or with organic reagents such as Pb(PDC)₂ [6] (PDC, pyrrolidine dithiocarbamate), Co(II)-APDC [7] and Mn[II]-DDTC [8]; (c) adsorption onto solid-phases such as chelating resin [9], [10], activated alumina [11] and/or Ettringite [12].

Liquid–liquid extraction prior to the instrumental analysis of trace metal provides a method of improving the sensitivity and reducing the interference from the matrix. Various organic solvents such as benzene, 4-methyl-2-pentanone and chloroform have been used in the conventional solvent extraction however, most of them are toxic materials affecting on health and environment. Therefore, it is necessary to decrease their utilization and discharge to the environment. Foy and Pacey [13] have reported the supercritical fluid extraction for the speciation of chromium. The extraction method by the supercritical carbon dioxide offer several advantages including extraction without an organic solvent, adaptability of the method to a metal valence speciation over the traditional solvent extraction.

Recently the use of water-based two-phase system is increasing because of its advantages over liquid–liquid extraction with water immiscible organic solvents. Aqueous two-phase system provides milder extraction conditions for biomaterials due to its unnecessary of organic solvent. Our aqueous two-phase system consisting of tetrabutylammonium bromide (TBAB) aqueous solution and (NH₄)₂SO₄ have been reported to be useful for the extraction of sodium copper chlorophyllin and cadmium ion [14]. We have shown that cadmium can be selectively extracted from large amounts of zinc into the TBAB-rich upper phase.

In the present paper we have investigated the distribution behavior and extraction mechanism of Cr(VI) in TBAB-(NH₄)₂SO₄ aqueous two-phase system. The method described here was also applied to the determination of Cr(VI) in the waste water samples.