XAFS Studies of Nickel and Sulfur Speciation in Residual Oil Fly-Ash Particulate Matters (ROFA PM)

XAFS spectroscopy has been employed to evaluate the effect of fuel compositions and combustion conditions on the amount, form, and distribution of sulfur and nickel in size-fractionated ROFA PM. Analysis of S K-edge XANES establish that sulfate is abundant in all PM. However, depending upon the combustion conditions, lesser amounts of thiophenic sulfur, metal sulfide, and elemental sulfur may also be observed. Least-squares fitting of Ni K-edge XANES reveals that most of the nickel in PM is present as bioavailable NiSO₄.nH₂O. The insoluble Ni mainly exists as a minor species, as nickel ferrite in PM_2.5 (PM < 2.5 μm) and nickel sulfide, Ni_xS_y in PM_2.5+ (PM > 2.5 μm). The Ni K-edge XANES results are in agreement with the EXAFS data. Such detailed speciation of Ni and S in PM is needed for determining their mobility, bioavailability, and reactivity, and hence, their role in PM toxicity. This information is also important for understanding the mechanism of PM formation, developing effective remediation measures, and providing criteria for identification of potential emission sources. Transition metals complexing with sulfur is ubiquitous in nature. Therefore, this information on metal sulfur complex can be critical to a large body of environmental literature.