Liquid-phase microextraction method for the separation of hexavalent and total chromium in environmental water samples with spectrophotometric detection

Abstract

Metal pollution, particularly chromium, in water and food samples is a critical issue due to its transfer to the human body through the food chain and its threat to human health. Among the chromium species that can be found in water samples, chromates are classified as toxic by scientific authorities. Spectroscopic instruments have limitations in metal speciation analysis, and there is a need for suitable methods that allow chromium speciation. In this work, a simple microextraction method was presented to extract Cr(VI) into an extraction solvent after it was complexed with a cationic reagent named benzyldimethyltetradecylammonium chloride, followed by measurement using ultraviolet-visible spectrometry. Total chromium was obtained after the oxidation of Cr(III) to Cr(VI) using a sulfuric acid/potassium permanganate/potassium oxalate combination. The method allows for a calibration curve (0.015–0.35 mg L⁻¹) and a low detection limit (1.8 μg L⁻¹) for the quantification of Cr(VI) and total Cr in seawater and wastewater samples. The accuracy of the proposed method was evaluated by the spiking-recovery method, with recoveries in the range of 91–103% for real water samples, and a certified reference material (LGC6026) was analyzed using the method for the determination of chromium content. In this work, benzyldimethyltetradecylammonium chloride (also known as the zephiramine ion, a disinfectant and antimicrobial agent commonly used in medical treatments) is used for the first time in a microextraction method for complexing with Cr(VI). The combination of microextraction with ultraviolet-visible spectrometry also makes this method low-cost and easy to apply in general chemistry laboratories. In addition, the presented method provides a more convenient oxidation procedure from Cr(III) to Cr(VI) in acidic media, using potassium oxalate instead of sodium azide to remove excess potassium permanganate.