Nickel Speciation Of Residual Oil Ash

EPA Grant Number: R827649C002
Subproject: this is subproject number 002 , established and managed by the Center Director under grant R827649
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Center for Air Toxic Metals® (CATM®)
Center Director: Groenewold, Gerald
Title: Nickel Speciation Of Residual Oil Ash
Investigators: Galbreath, Kevin C. , Huffman, Gerald P. , Huggins, Frank E. , Toman, Donald L. , Won, John , Zygarlicke, Christopher J.
Institution: University of North Dakota
EPA Project Officer: Chung, Serena
Project Period: October 15, 1999 through October 14, 2002
Project Amount: Refer to main center abstract for funding details.
RFA: Center for Air Toxic Metals (CATM) (1998) RFA Text |  Recipients Lists
Research Category: Targeted Research


The speciation of Ni emitted from residual-oil-fired utility boilers requires investigation because the possible presence of small respirable particles containing nickel subsulfide (Ni3S2) is a health concern. An experimental approach was used to investigate the Ni speciation of residual oil combustion ash. Ash from a low- and high-sulfur (0.33 and 1.80 wt%, respectively) residual oil was produced using a laboratory-scale combustion system at excess O2 concentrations of I 1 and 2 or 3 mol%. Ni speciation analyses were performed using XAFS spectroscopy, sequential extraction-ASV, and CPEV.

Research goals are threefold:

  • Identify and quantify the chemical forms of Ni in No. 6 fuel oil combustion ash
  • Investigate the effects of fuel sulfur (S) and excess 0, concentrations on Ni speciation
  • Evaluate the comparability of Ni speciation analysis results obtained by sequential extraction-ASV, CPEV, and XAFS spectroscopy


Combustion tests were conducted using a CEPS, a 42-MJ/hr combustion system, to elucidate differences in the Ni speciation of ashes produced from low- and high-S fuel oils at excess O2 concentrations of I 1 and 2 or 3 mol%. Detailed descriptions of the CEPS are provided in the CATM 1994-1995 Annual Report [l] and in a previous CATM Newsletter [2]. Bulk ash samples were collected at the inlet of the convection pass section of the CEPS using glass-fiber filters. The ash samples were analyzed first using XAFS spectroscopy because it is a nondestructive technique. Ni K-edge XAFS spectroscopy measurements were conducted on beam line X-19A of the National Synchrotron Light Source, Brookhaven National Laboratory, New York. XAFS spectra of reagent-grade Ni compounds were acquired and used essentially as "fingerprints" for identifying different Ni species. The five-step extraction procedure in the table below was also used for determining Ni speciation. Ni in each extract was quantified by ASV of nickel dimethylglyoxime collected on a hanging mercury drop electrode with a CH-620 electroanalytical system in square-wave voltammetry mode. Ni concentration was obtained by the method of standard additions. In addition to analyzing the Ni extraction fractions, total Ni was determined. Although this method currently cannot be used to discriminate between nickel sulfide and subsulfide species, CPEV was used to distinguish between Ni3S2 and NiS. Mineral and synthetic standards of Ni3S2 and NiS as well as ash and extraction residues were mixed with microcrystalline graphite powder and fashioned into CPEs. Cyclic voltammetry and linear sweep voltammetry were performed using a CH-620 electroanalyzer to identify Ni3S2 and NiS.


XAFS spectroscopy, sequential extraction-ASV, and CPEV measurements indicate that soluble NiSO4 and not the more toxic Ni3S2 ,species predominate in low- and high-S residual oil ashes produced experimentally at excess 0, concentrations of 5 1% and 2 or 3 mol%. As indicated in the following table, the sequential extraction-ASV method detected the presence of significant proportions of NiO that was not detected using XAFS spectroscopy, even though the spectra of NiO are very distinctive from those for the dominant NiSO4 species. In future research, residue from the first step of the extraction procedure in the table on p. 44 will be analyzed using XAFS spectroscopy to corroborate the existence of NiO in residual oil ash. Fuel S content did not significantly affect Ni speciation; however, increasing excess 02 concentrations promoted Ni sulfation. The sequential extraction-ASV method also indicated the presence of very small proportions, <2%, of nickel sulfide. Analyses of a high-sulfur oil ash sample and the extraction residues from this ash by CPEV suggest that the nickel sulfide is present as NiS and not Ni3S2. The proportions of sulfidic nickel (NixSy) and NiO measured in these ash samples produced experimentally are much lower, while the relative proportions of soluble Ni are much greater than previous sequential extraction-ASV measurements of oil ashes collected from utility-scale boilers. Potential sources of these differences in Ni speciation include the sampling methods employed and the actual physicochemical properties of the ashes produced in a laboratory-scale versus utility-scale combustion system. Research is currently under way to identify the cause(s) of disagreement in Ni speciation results.


Knowledge of the chemical speciation of Ni in the ash emitted from oil-fired boilers is important with regard to potential human exposure and adverse health effects. Specifically, the presence of small respirable particles containing Ni3S2 is of primary concern from a human health standpoint. The chemical speciation of Ni emissions has been investigated recently using a modified EPA Method 5 sampling train in conjunction with a sequential extraction-ASV method. This method is based on treating a relatively small (1 O-l 00 mg) but representative ash sample to successive leaching steps to separate the analyte Ni species from the sample matrix. Ni in the extracted fractions is then analyzed using ASV, an electroanalytical technique. Ni speciation results from this method suggest that as much as -25% of the total Ni emitted from oil-fired utility boilers occurs as a sulfide or subsulfide phase (e.g., NiS2, Ni3S4, NiS, Ni7S6, Ni3S2) while the remaining 75% is apportioned among NiO, soluble Ni species (e.g., NiSO4, NiCl2, and NiCO3), nickel silicate(s), and elemental nickel (Ni0). The capabilities of CPEV and XAFS spectroscopy to directly determine Ni speciation are being used to evaluate these sequential extraction-ASV speciation results. Although extraction methods determine speciation indirectly, they are necessary to make trace element speciation determinations more quantitative and readily available.

Supplemental Keywords:

Scientific Discipline, Air, Toxics, Chemical Engineering, Environmental Chemistry, HAPS, Environmental Monitoring, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, residual oil-fired utility boiler, nickel speciation, oil fired utility boiler, nickel & nickel compounds, Nickel Compounds, residual oil ash

Progress and Final Reports:

  • 2000
  • 2001
  • Final

  • Main Center Abstract and Reports:

    R827649    Center for Air Toxic Metals® (CATM®)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R827649C001 Development And Demonstration Of Trace Metals Database
    R827649C002 Nickel Speciation Of Residual Oil Ash
    R827649C003 Atmospheric Deposition: Air Toxics At Lake Superior
    R827649C004 Novel Approaches For Prevention And Control For Trace Metals
    R827649C005 Wet Scrubber System
    R827649C006 Technology Commercialization And Education
    R827649C007 Development Of Speciation And Sampling Tools For Mercury In Flue Gas
    R827649C008 Process Impacts On Trace Element Speciation
    R827649C009 Mercury Transformations in Coal Combustion Flue Gas
    R827649C010 Nickel, Chromium, and Arsenic Speciation of Ambient Particulate Matter in the Vicinity of an Oil-Fired Utility Boiler
    R827649C011 Transition Metal Speciation of Fossil Fuel Combustion Flue Gases
    R827649C012 Fundamental Study of the Impact of SCR on Mercury Speciation
    R827649C013 Development of Mercury Sampling and Analytical Techniques
    R827649C014 Longer-Term Testing of Continuous Mercury Monitors
    R827649C015 Long-Term Mercury Monitoring at North Dakota Power Plants
    R827649C016 Development of a Laser Absorption Continuous Mercury Monitor
    R827649C017 Development of Mercury Control Technologies
    R827649C018 Developing SCR Technology Options for Mercury Oxidation in Western Fuels
    R827649C019 Modeling Mercury Speciation in Coal Combustion Systems
    R827649C020 Stability of Mercury in Coal Combustion By-Products and Sorbents
    R827649C021 Mercury in Alternative Fuels
    R827649C022 Studies of Mercury Metabolism and Selenium Physiology