Determinação de Al, Cr, Fe, Mn, Pb e Zn em amostras de óleo lubrificante automotivo usado por espectrometria de absorção atômica após extração por quebra de emulsão

Abstract

The presence of metallic species in used lubricating oil (LO) is an important indicator of the wearing process undergone by engines, and can lead to environmental contamination through vehicle exhaust system. Therefore, in this work two methods of sample preparation for the determination of Al, Cr, Fe, Mn, Pb and Zn in used LO samples by Flame Atomic Absorption Spectrometry (F AAS) were described. The first method is the acid digestion of samples on hot plate, using H2SO4, HNO3 and H2O2 as oxidizing agents, where the accuracy of the method represented by RSD values is minor than 10%, and the recovery test of the analytes ranged from 81.7 to 109.1%. However, the limits of quantification for this method were 33.3 mgKg-1 (Al), 16.0 mgKg-1 (Cr), 8.3 mgKg-1 (Fe), 6.7 mgKg-1 (Mn), 37.2 mgKg-1 (Pb) and 3.8 mgKg-1 (Zn), which makes this method not suitable for the termination of Al, Cr, Mn and Pb by F AAS on used LO samples. The second method was the extraction of the analytes by emulsion breaking (EBE), which consisted in breaking the water-in-oil emulsion formed when mixing the sample with an acid solution of Triton X-114 (extraction solution). The parameters optimized for the EBE process were: the composition and volume of the extraction solution, the emulsion breaking process, the agitation time and the use of an organic solvent that is used to decrease sample viscosity. The results obtained were: 1.5 mL of extraction solution (6.0% w/v Triton X-114, 2.1 molL-1 HNO3), emulsion breaking by heating at 80°C, 1.0 minute stirring and n-hexane as the organic solvent. The limits of quantification found for the EBE method were 0.700 mgkg-1 (Al), 0.351 mgkg-1 (Cr), 0.499 mgkg-1 (Fe), 0.102 mgkg-1 (Mn), 0.227 mg kg-1 (Pb) and 0.038 mgKg-1 (Zn). The method was applied to four different samples, where the content of the analytes followed the order Zn > Fe > Al > Mn > Pb > Cr. The similar tendency found for the analytes concentration on the different samples can indicate that the accumulation of those metallic species in LO samples comes from the same kind of metal alloys used in car engines.