A sensitive speciation analysis of chromium in natural and industrial effluents by electrothermal atomic absorption spectrometry after a novel ligandless mixed-micelle dispersive liquid–liquid microextraction

Abstract

A simple, fast and sensitive method was developed for determining Cr(III), Cr(VI) and total Cr separately in natural river and lake waters and tannery effluent samples. In this method, a new strategy of micelle and mixed-micelles was introduced in dispersive liquid–liquid microextraction (M-MM-DLLME) for sequential and simultaneous separation and pre-concentration of Cr(VI) and Cr(III) at a sampling site. Cationic Aliquat-336 and anionic dioctylsulfosuccinate (DOSS) surfactant aggregates were used as dispersive and chelating solvents, respectively, for extracting Cr(VI) and Cr(III). In sequential speciation, the hydrophilic HCrO₄⁻, first reacts electively with the cationic head group solubilising sites of Aliquat-336 micelles through electrostatic interaction and form extractable charge neutral Cr(VI)-Aliquat-336 at room temperature in the presence of Cr(III). The extracting trichloroethylene pre-concentrates this extractable species from bulk aqueous phase and leaves the unreacted cationic Cr(H₂O)₆³⁺ in the supernatant. This supernatant is subjected to a similar extraction procedure after adding an anionic DOSS surfactant. In the simultaneous speciation, both the species were extracted by adapting a similar extraction procedure after adding DOSS and Aliquat-336 mixed-micelles to fresh aliquots to determine total Cr, avoiding the addition of a chelating agent and oxidation/reduction steps. The surfactant stabilized chromium species in the trichloroethylene phases were transferred to a laboratory for quantification using electrothermal atomic absorption spectrometry (ETAAS) the parameters affecting the process are evaluated and optimized. Under the optimized conditions, the pre-concentration factors and limits of detection for Cr(III) and Cr(VI) in sequential and total Cr in simultaneous extractions were 100, 90 and 83 and 0.5, 0.6 and 0.7 pg mL⁻¹, respectively. The accuracy of the procedure was verified by comparing the sum of the concentrations of each individual species obtained from the sequential extraction with total Cr obtained in the simultaneous extraction procedures and applied to certified reference materials such as BCR 714 industrial influent, BCR 715 industrial effluent and NIST 1640 natural water. This method was applied to on-site extraction of individual and total Cr from natural waters and effluents.