Detection and Estimation of Re-Refined Engine Oil Bottoms in Asphalt Binders: Texas Department of Transportation’s Approach with Wavelength Dispersive X-Ray Fluorescence Spectroscopy

Abstract

The Texas Department of Transportation investigated the use of wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy to estimate the amount of re-refined engine oil bottoms (REOB) in asphalt binders. The objectives of this investigation were to use WDXRF to detect and quantify REOB in binder samples and to evaluate the accuracy of the quantified content. The detection of REOB in asphalt binders was determined by the presence of the elements phosphorus, molybdenum, zinc, copper, calcium, and potassium. Quantifying REOB content was achieved by determining the intensity of these elements and correlating that intensity with a known dosage of REOB with the use of a calibration standard. More than 100 binder and REOB combinations were prepared, and four calibration standards were developed for this study. The data analysis determined that correlations were specific to binder and REOB combinations. Significant error was introduced into the analysis when the REOB content for a sample was quantified according to a calibration curve that did not include the same binder and REOB. In one case, as much as a 92.0% error was introduced by that approach. To reduce this error and provide a reliable method, vanadium (V) and sulfur (S) intensities were used to select the appropriate calibration standard. Error was reduced by as much as half or more when the REOB content was quantified on the basis of selecting a calibration standard with similar S and V intensities.