Ultratrace determination and speciation of hexavalent chromium by EDXRF and TXRF using dispersive micro-solid phase extraction and tetraethylenepentamine graphene oxide

Hexavalent chromium is much more toxic than trivalent chromium and is severe environmental pollution caused by human activity. The presence of Cr(VI) ions in waters comes from anthropogenic sources, mainly from industries, and poses an enormous danger. Because of the health effects of Cr(VI) ions on humans, even at very low concentrations, it is necessary to control its levels in the water. However, the determination and speciation of Cr (VI) in water samples remains a sophisticated subject, and according to the WHO recommendation, further studies on reliable and validated methods should be continued. In this study, graphene oxide (GO) was modified with tetraethylenepentamine (TEPA) for the highly effective adsorption and determination of Cr(VI) ions by energy-dispersive X-ray fluorescence (EDXRF) and total-reflection X-ray fluorescence spectrometry (TXRF). The experiment shows that Cr(VI) ions can be adsorbed from aqueous solutions at pH 3.5 with a maximum adsorption capacity of 102 mg g-1 using minimal adsorbent doses, 10-50 mu g mL-1, much lower than those of the currently reported adsorbents. These adsorptive properties of GO-TEPA and selectivity toward Cr(VI) in the presence of Cr (III) indicate its potential use as a micro-adsorbent in the determination and speciation of chromium. Due to the high preconcentration factors of 865 and 100, for EDXRF and TXRF, respectively, and high recovery of 98.5-100%, the method based on dispersive micro-solid phase extraction allows obtaining extremely low detection limits of 53 and 3.5 pg mL-1 for EDXRF and TXRF. The exceptional adsorptive properties of GO-TEPA, including the possibility of application in micro-quantities, allow for the development of the ultra-trace method according to the fundamental principles of green analytical chemistry. It significantly expands the possibilities of using the EDXRF, as well as the TXRF technique in water analysis.