Quantitative Speciation of Mn-Bearing Particulates Emitted from Autos Burning (Methylcyclopentadienyl)manganese Tricarbonyl-Added Gasolines Using XANES Spectroscopy

Abstract

The chemical nature of Mn-containing particulates emitted from (methylcyclopentadienyl)manganese tricarbonyl-added gasoline engines has been elucidated using Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy. Edge shift data from the X-ray absorption near-edge structure (XANES) spectra showed that the average Mn valence in these particulates is ∼2.2. Using a principal component analysis (PCA) algorithm, the number and type of probable species contained in these particulates were determined to be three, consisting of Mn3O4, MnSO4·H2O, and a divalent manganese phosphate, Mn5(PO4)[PO3(OH)]2·4H2O. The proportions of these Mn phases in each particulate sample were evaluated quantitatively using least-squares fitting (LSF) of the experimental XANES spectra with linear combinations of these principal component (model compound) spectra. Two groups of Mn-bearing particulates may be distinguished: group I having 4−9 wt % of Mn3O4 and exhibiting a single intense first major absorption maximum at the Mn K-edge and group II containing 15−22 wt % of Mn3O4 and exhibiting a doublet absorption maximum at lower intensity. Fourier transforms of the EXAFS signals were found to corroborate the XANES results. This study clearly establishes XANES spectroscopy, in combination with PCA and LSF, as a quantitative analytical tool for speciation of dilute and/or amorphous multicomponent environmental materials not easily attainable with conventional methods.