Cloud point microextraction involving graphene oxide for the speciation of very low amounts of chromium in waters

Highlights

• Chromium (III) is retained on graphene oxide.

• Graphene oxide is transferred to the coacervate obtained of Triton X-45.

• Chromium is measured by electrothermal atomic absorption spectrometry.

• The reduction of chromium (VI) allows speciation at a very low level.

Abstract

A graphene oxide (GO) suspension was used to retain chromium (III) species in a neutral solution. The addition of Triton X-45 followed by heating resulted in a coacervate, to which the GO nanoparticles containing the analyte retained were transferred. Chromium was determined directly in the surfactant-rich phase by electrothermal atomic absorption spectrometry. The procedure was repeated using a second aliquot of sample to which sodium hydrogen sulphite was added to reduce chromium (VI) to chromium (III). By again obtaining the analytical signal of chromium in the condensed phase, the concentration of the hexavalent form was calculated by difference. Using 10 mL for the sample volume and 0.01% for the Triton X-45 concentration, the enrichment factor was 140, and the detection limit was 5 ng L⁻¹. The repeatability for five consecutive measurements at the 100 ng L⁻¹ level was close to 5%. The reliability of the procedure was verified by analysing seven water samples with certified contents of total chromium. Speciation data are given for these standard materials and for seven potable waters and one seawater sample.

Introduction

Solid phase extraction is a widely used methodology considered appropriate for sample preparation and preconcentration purposes in the analytical laboratory. It is of even greater interest if the sorbent has a low particle size since the surface area available for the extraction stage considerably increases, and the process can be carried out with very small amounts of sorbent. Hence, among other advantages, the benefits inherent to microextraction approaches are fully exploited [1], [2]. This is the case of graphene [3], [4] and related materials. Graphene oxide (GO), which is obtained by chemical oxidation of graphite, has oxygen-containing functional groups (hydroxyl, epoxide and carboxylic acid) [5] which can retain polar chemical species [6], [7], [8] and facilitate dispersion to give stable aqueous colloids [9]. The first characteristic means that GO retains metallic species in solution as they are charged positively [1], [3], [10], [11], [12], [13], [14], [15], [16], but the second hinders application from an analytical point of view since separating the solid containing the retained analyte from the liquid is difficult, even after prolonged centrifugation. If the material is placed in small columns, high pressures will result from the low particle size [17]. Alternatives, such as aggregation of the GO dispersion by adding electrolytes [9], [17], [18], [19], changing pH [18] or the incorporation of GO to a magnetic core [15], [20], [21], [22] have been studied but the analytical potential these particles have has not been yet fully exploited.

Because of its hydrophilic and hydrophobic properties, GO could be incorporated in the coacervates obtained from solutions containing surfactants in a cloud point extraction (CPE) [23], [24], [25]. If this interaction occurs, the transfer to the coacervate of GO with the retained metallic species should result in a separation with a high enrichment factor due to the low volume of the condensed phase recovered in CPE. If, in addition, a sensitive analytical technique that operates with low-volume aliquots, such as electrothermal atomic absorption spectrometry (ETAAS), is used to measure the metallic ion, the resulting analytical procedure should have great sensitivity. To the best of our knowledge, this analytical possibility has not been exploited. This paper describes studies carried out to optimize a new procedure for the determination of very low concentrations of chromium in waters using GO, CPE and ETAAS. Although the literature reports analytical procedures for chromium involving GO, the nanoparticles are used from a different perspective [17], [26], [27], [28], and no benefit is obtained from the simplicity that CPE offers in the analytical laboratory, as shown in recent studies [24], [25].