Sequential extraction procedure for the separation of Ni and V species in crude oil and analysis by ETAAS, GC–MS, and IR

Abstract

The determination of nickel and vanadium in crude oil has been evaluated in several scientific studies because these elements can contribute to equipment corrosion, the contamination of catalytic processes, and environmental pollution. Many studies have focused on the evaluation of the total concentration of these elements. However, although it is important to know the total elemental concentration to determine the chemical composition of a sample, this does not yield information about the amount of potentially toxic compounds that may be associated with different macromolecules present in the sample. Here, we report the development of a sequential extraction procedure for the separation of nickel and vanadium species. To determine the total concentration of Ni and V, samples, as well as a certified reference material (NIST – SRM 1634C), were acid digested and analyzed by electrothermal atomic absorption spectrometry (ETAAS). The separation of the species was performed by column chromatography with silica gel as the stationary phase, and hexane (fraction 1), toluene (fraction 2), dichloromethane (DCM) + methanol (fraction 3), and methanol (fraction 4) as the eluents. The extracts obtained in each fraction were analyzed by ETAAS, gas chromatography–mass spectrometry (GC–MS), and infrared (IR) measurements. The experimental results showed that the total concentrations of the elements in the crude oil were between 7.4 and 14.3 µg g−1 for Ni and 9.8 to 14.6 µg g−1 for V. The analysis of SRM 1634C showed that 89% of Ni and 103% of V were recovered. The GC–MS analysis showed the following elution order: saturated compounds and aromatics (fraction 1), aromatic and cyclic compounds (fraction 2), aromatic compounds (fraction 3), and polar compounds (fraction 4). The results obtained by ETAAS showed that 100% of the Ni and V contained in crude oil can be separated in three different extracts. However, the IR spectra and GC–MS analysis allowed us to conclude that Ni and V are associated with compounds containing nitrogen and oxygen that were not identified by gas chromatography.