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NICKEL SPECIATION ANALYSES 

OF RESIDUAL OIL FLY ASHES 

USING X-RAY TECHNIQUES

Product ID #070441

Final Report, December 2010
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University of North Dakota Energy & Environmental Research Center

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Copyright © 2010 Electric Power Research Institute, Inc. All rights reserved.

This publication is a corporate document that should be cited in the literature in the following 

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Nickel Speciation Analyses of Residual Oil Fly Ashes Using X-Ray Techniques. EPRI, Palo Alto, 

CA: 2010.

iii

ACKNOWLEDGMENTS

The following organization(s), under contract to the Electric Power Research Institute (EPRI), 

prepared this report:

University of North Dakota, Energy & Environmental Research Center

15 North 23rd Street, Stop 9018

Grand Forks, ND 58202-9018

Principal Investigator

K. Galbreath

University of Kentucky

103 S. J. “Sam” Whalen Bldg

533 S. Limestone Street

Lexington, KY 40506-0043

Principal Investigator

F. Huggins

This report describes research sponsored by EPRI.

v

ABSTRACT

Representative duplicate fly ash samples were obtained from the stacks of two 50-MW (Units 9 

and 6 of the Honolulu and Waiau power plants, respectively) and one 80-MW (Unit 1 of the 

Kahe power plant) utility boilers using a modified U.S. Environmental Protection Agency 

Method 17 sampling train assembly as 0.3 to 0.4 wt% sulfur residual (No. 6 fuel) oils were 

burned during full-load power plant operations. Residual oil fly ash (ROFA) samples were 

analyzed for Ni speciation using x-ray absorption fine structure (XAFS) spectroscopy and x-ray 

diffraction (XRD). ROFA deionized H2O extraction residues were also analyzed for Ni 

speciation using XAFS and XRD. Total Ni concentrations in the ROFAs ranged from 2.4 to

4.8 wt%. Nickel speciation analysis results indicated that 80% to 95% of the total Ni present in 

the ROFAs was NiSO4.6H2O with subordinate proportions, ≤ 20% of the total Ni, present as a 

Ni-containing spinel compound, similar in composition to NiFe2O4

Keywords

and/or NiO. XAFS and XRD 

analyses did not detect any sulfidic Ni compounds; the XAFS detection limit was 3% of the total 

Ni concentration.

Nickel, nickel emissions, nickel speciation, residual (No. 6 fuel) oil, x-ray absorption fine 

structure spectroscopy, x-ray diffraction, residual oil fly ash

vii

CONTENTS

1 INTRODUCTION ..................................................................................................................1-1

2 BACKGROUND....................................................................................................................2-1

XAFS Spectroscopy ............................................................................................................2-1

Power Plant Sites ................................................................................................................2-2

3 EXPERIMENTAL..................................................................................................................3-1

Residual Oil Analyses..........................................................................................................3-1

Fly Ash Stack-Sampling and Analysis Methods...................................................................3-1

Soluble Ni Extraction ......................................................................................................3-1

Ni XAFS Spectroscopy ...................................................................................................3-2

XRD................................................................................................................................3-4

4 RESULTS AND DISCUSSION..............................................................................................4-1

Residual Oil Analyses..........................................................................................................4-1

ROFA Ni Analyses ..............................................................................................................4-1

EPA Method 29...............................................................................................................4-1

Ni XAFS Spectroscopy ...................................................................................................4-2

XANES Pre-Edge Systematics .....................................................................................4-11

Least-Squares Fitting of Ni XANES and EXAFS Spectra..............................................4-14

XRD Analyses ..............................................................................................................4-16

5 CONCLUSIONS ...................................................................................................................5-1

6 REFERENCES .....................................................................................................................6-1

A APPENDIX..........................................................................................................................A-1

Continued . . .
viii

CONTENTS Continued

PHOTOGRAPHS OF QUARTZ THIMBLE FILTERS ..........................................................A-1

B APPENDIX..........................................................................................................................B-1

X-RAY DIFFRACTOGRAMS OF BULK ROFA AND NI EXTRACTION RESIDUES ...........B-1
ix

LIST OF FIGURES

Figure 2-1 XAFS Spectrum of Elemental Selenium Showing the Division of the Spectrum 

into Separate XANES and EXAFS Regions and the Subsequent Generation of a

Radial Structure Function (RSF) by Fourier Transform of the EXAFS Region of the 

Spectrum..........................................................................................................................2-1

Figure 3-1 Photograph of Deposits on Quartz Thimble Filters from HPP 9...............................3-3

Figure 4-1 Ni XAFS Spectra Collected at SSRL for Thimble Filter Samples from HPP 

Unit 9................................................................................................................................4-2

Figure 4-2 Ni XAFS Spectra Collected at NSLS for Thimble Filter Samples from KPP 

and WPP Units 1 and 6, Respectively ..............................................................................4-3

Figure 4-3 Ni XAFS Spectra of Duplicate ROFA Samples and a ROFA Residue Sample 

from the Stack of HPP Unit 9; (a) XANES Spectra, (b) Derivative XANES Spectra, 

(c) EXAFS (k3

chi) Spectra, and (d) RSF Spectra..............................................................4-5

Figure 4-4 Ni XAFS Spectra of Duplicate ROFA Samples from the Stacks of KPP and 

WPP Units 1 and 6, Respectively; (a) XANES Spectra, (b) Derivative XANES 

Spectra, (c) EXAFS (k3

chi) Spectra, and (d) RSF Spectra................................................4-6

Figure 4-5 Ni XAFS Spectra of ROFA Residues Corresponding to KPP and WPP Units 1 

and 6, Respectively; (a) XANES Spectra, (b) Derivative XANES Spectra, (c) EXAFS 

(k3

chi) Spectra, and (d) RSF Spectra................................................................................4-7

Figure 4-6 Ni XAFS Spectra of NiSO4, NiSO4·H2O, and NiSO4·6H2O; (a) XANES 

Spectra, (b) Derivative XANES Spectra, (c) EXAFS (k3

chi) Spectra, and (d) RSF 

Spectra.............................................................................................................................4-8

Figure 4-7 Ni XAFS Spectra of Ni3S2, NiS, and NiS2; (a) XANES Spectra, (b) Derivative 

XANES Spectra, (c) EXAFS (k3

chi) Spectra, and (d) RSF Spectra...................................4-9

Figure 4-8 Ni XAFS Spectra of NiO, NiFe2O4, and NiAl2O4; (a) XANES Spectra, 

(b) Derivative XANES Spectra, (c) EXAFS (k3

chi) Spectra, and (d) RSF Spectra...........4-10

Figure 4-9 Comparison of the Normalized Height of the Pre-Edge Inflection Feature at 

8333 eV in Ni Derivative XANES Spectra for Ni Compounds..........................................4-12

Figure 4-10 Comparison of the Normalized Height of the Pre-Edge Inflection Feature at 

8333 eV in Derivative XANES Spectra of ROFA Sampled from the Three Hawaiian 

Units...............................................................................................................................4-12

Figure 4-11 Examples of Least-Squares Fits to EXAFS Spectra of ROFA Samples: Bulk 

ROFA Sample, K1-Ni-2 (left), and ROFA H2O Extraction Residue, K1-Ni-1R (right) ......4-15

Figure A-1 Photograph of a Thimble Filter with ROFA Sampled from KPP Unit 1 ...................A-1

Continued . . .
x

LIST OF FIGURES Continued

Figure A-2 Photograph of a Duplicate Thimble Filter with ROFA Sampled from KPP 

Unit 1...............................................................................................................................A-2

Figure A-3 Photograph of a Thimble Filter with ROFA Sampled from WPP Unit 6 ..................A-2

Figure A-4 Photograph of a Duplicate Thimble Filter with ROFA Sampled from WPP 

Unit 6...............................................................................................................................A-3

Figure B-1 X-ray Diffractogram of ROFA Sample from HPP Unit 9 Indicating Spectral 

Matches with NiSO4·6H2O and CaSO4 ...........................................................................B-1

Figure B-2 X-ray Diffractogram of Duplicate ROFA Sample from HPP Unit 9 Indicating 

Spectral Matches with NiSO4·6H2O and CaSO4 .............................................................B-2

Figure B-3 X-ray Diffractogram of ROFA Sample from KPP Unit 1 Indicating Spectral 

Matches with NiSO4·6H2O, NiFe2O4, and CaSO4 ...........................................................B-3

Figure B-4 X-ray Diffractogram of Duplicate ROFA Sample from KPP Unit 1 Indicating 

Spectral Matches with NiSO4·6H2O, NiFe2O4, and CaSO4 .............................................B-4

Figure B-5 X-ray Diffractogram of ROFA Sample from WPP Unit 6 Indicating Spectral 

Matches with NiSO4·6H2O and CaSO4 ...........................................................................B-5

Figure B-6 X-ray Diffractogram of Duplicate ROFA Sample from WPP Unit 6 Indicating 

Spectral Matches with NiSO4·6H2O and CaSO4 .............................................................B-6
xi

LIST OF TABLES

Table 2-1 Power Plant Descriptions and Abbreviations ............................................................2-2

Table 3-1 List of Samples Cross-Referenced to XAFS File Names..........................................3-2

Table 3-2 List of ROFA H2

3-2

O Extraction Residue Samples Cross-Referenced to XAFS 

File Names .......................................................................................................................

Table 4-1 Residual Oil Proximate and Ultimate Analysis Results .............................................4-1

Table 4- 2 Average ROFA Ni Concentrations and Emission Rates ..........................................4-2

Table 4-3 Ni Contents of the Filter Blank Samples from KPP and WPP Units 1 and 6 .............4-4

Table 4-4 Estimates of NiS and Ni3S2 in ROFA from Three HECO Units...............................4-14

Table 4-5 Results of Least-Squares Fitting of Combined Bulk ROFA and Corresponding 

ROFA H2O Extraction Residue Spectra to Spectra for NiSO4·6H2O and NiO ................4-15

Table 4-6 Results of Least-Squares Fitting of EXAFS Spectra (3–10 Å-1) for Bulk ROFA 

and ROFA H2O Extraction Residues to Linear Combinations of the Corresponding 

Spectra for NiSO4·6H2O, NiFe2O4, and NiO ..................................................................4-16

Table 4-7 Bulk ROFA Phase Assemblages............................................................................4-17

Table 5-1 Ni Species Emission Estimates................................................................................5-1

1-1

1

INTRODUCTION

Nickel is a transition metal that occurs throughout the environment and is discharged into air, 

water, and soil from various natural and industrial sources. A significant natural source of 

atmospheric Ni is windborne dust particles derived from the weathering of rocks and soils and 

from volcanic eruptions [1]. The average Ni concentration of crustal rocks is 75 ppm [2].

Although Ni is a ubiquitous metal, industrialization has increased its flux into the environment

[3]. Anthropogenic stationary sources that release Ni into ambient air include 1) combustion and 

incineration sources (oil- and coal-burning units in utility, industrial, and residential use sectors 

and medical, municipal, and sewage sludge incinerators), 2) high-temperature metallurgical 

operations (steel and Ni alloy manufacturing, secondary metals smelting, and coproduct Ni 

recovery), 3) primary production operations (mining, milling, smelting, and refining), and 

4) chemical and catalyst sources (Ni chemical manufacturing, electroplating, Ni–Cd battery 

manufacturing, and catalyst production, use, and reclamation) [4, 5]. The mobile source 

contribution to Ni emission inventories is small and derived primarily from engine wear and 

impurities in engine oil and fuel additives [6]. However, commercial marine vessels are 

significant mobile sources of Ni in areas near harbors [7, 8].

In the United States, the impetus for focusing on individual elements, such as Ni, in air pollution 

derives from the 1990 Clean Air Act Amendments (CAAA) [9] and the attainment of National 

Ambient Air Quality Standards [10]. Title III of the CAAA identifies 188 chemicals, including 

Ni and 15 other inorganic trace elements (As, Be, Cd, Cl, Co, Cr, F, Hg, Mn, P, Pb, Sb, Se, Th, 

and U) as potential hazardous air pollutants (HAPs) or air toxics. The U.S. Environmental 

Protection Agency’s (EPA’s) Integrated Urban Air Toxics Strategy also classifies Ni as an urban 

HAP [11]. Many stationary sources have had to report Ni emissions as part of the EPA Toxics 

Release Inventory (TRI) [12]. Although TRI and similar reporting provide estimates of the 

amounts of Ni released into the environment, they are not an indicator of toxicity because the 

acute, chronic, and cancer-causing effects vary significantly for the different chemical species of 

Ni. For example, nickel subsulfide (Ni3S2) is considered the most carcinogenic Ni species on the 

basis of available human epidemiology and animal studies [5, 13, 14]. In contrast, inhalation 

exposure to water-soluble Ni salts alone, such as nickel hexahydrite (NiSO4 · 6H2O), have not 

been shown to cause cancer in animal studies [15–17]. Soluble Ni compounds, however, are 

more toxic to the respiratory tract than less soluble compounds. Insoluble nickel oxide 

compounds such as Ni-containing spinels (e.g., NiFe2O4), although still under investigation, are 

expected to have relatively low cancer potencies compared to Ni3S2 and may not be 

bioaccessible [5, 13, 14, 18]. Therefore, determining the Ni speciation of emission sources is of 

the utmost importance for assessing the inhalation health risks associated with airborne Nicontaining particles.
INTRODUCTION

1-2

The combustion of petroleum fuels (distillate fuel oils, residual fuel oil, jet fuel, kerosene, 

petroleum coke [converted to liquid petroleum], and waste oil) for generating steam and 

electricity is a significant anthropogenic source of Ni release to the atmosphere. Petroleum-fired 

capacity represented 1.1% (46,000,000 MWh) of the total U.S. electric utility capacity in 2008

[19]. Residual (No. 6 fuel) oil, a by-product from the petroleum-refining industry, is the most 

widely used for generating steam and electricity because of its relatively low cost compared with 

that of lighter oils. Residual oil production in the United States has declined since the late 1980s, 

but remains substantial at ≈ 42,000 barrels per month [20]. In addition to the electric utility 

industry, residual oil is an important fuel for marine and industrial boilers.

The fly ash produced from residual oil combustion (i.e., residual oil fly ash [ROFA]) contributes 

to ambient particulate matter pollution. Controlling fly ash emissions, however, is difficult 

because a substantial fraction of residual oil ash particles is generally submicrometer, 

corresponding to the size range of 0.1–1 μm, where existing industrial gas-cleaning devices are 

least effective [21–23]. As part of the EPA Study of Hazardous Air Pollutant Emissions from 

Electric Utility Steam Generating Units—Final Report to Congress [14], the inhalation health 

risks associated with HAP emissions from oil-fired utility boilers were estimated. EPA estimated 

that the contribution of Ni to the maximum individual risk exceeded that from all other HAPs by 

about an order of magnitude primarily because of its relatively high concentration, generally 1–

4 wt% Ni, in ROFA and known carcinogenic potency when in a Ni3S2 form. In 2004, EPA 

proposed Ni emission limits of 210 lb/1012 Btu on an input basis or 0.002 lb/MWh on an output 

basis for residual oil-fired electric utility units [24].

As highlighted in EPA’s study [14] and proposed emission limit ruling [24], the speciation of Ni 

emissions from oil-fired utility boilers is uncertain and requires additional investigation. Total Ni 

emissions were measured at 13 of the 149 power plants that burned oil in 1994, and speciation 

measurements were conducted at seven of them. The limited Ni speciation analyses performed 

indicate that 3%–26% of the total Ni emissions were composed of sulfidic Ni, although it is 

unknown whether Ni3S2 was present because of the limitations of the indirect (i.e., operationally 

defined) speciation method employed, sequential Ni extraction [25–28]. Sequential Ni extraction 

methods are based on treating a small but representative sample of fly ash or particulate matter to 

successive leaching steps to separate analyte Ni species from the sample matrix. Ni species 

concentrations in the extracted fractions are then determined using sensitive analytical 

techniques, such as anodic stripping voltammetry, graphite furnace atomic absorption 

spectroscopy (GFAAS), and inductively coupled plasma–atomic emission spectroscopy (ICP–

AES). Sequential extractions provide an operationally defined (e.g., based on particle size, 

solubility, bonding, and reactivity differences) determination of Ni forms (soluble, sulfidic, 

metallic, and oxidic) that is susceptible to errors because of inefficient and nonselective 

extractions and subsequent Ni redistribution effects [29]. In contrast, Galbreath et al. [30, 31]

used definitive speciation techniques, x-ray diffraction (XRD) and x-ray absorption fine structure 

(XAFS) spectroscopy, to identify the Ni species occurring in ROFAs sampled from 385- and 

400-MW utility boiler stacks. Direct speciation measurements indicated that >95% of the total Ni 

in ROFAs was present as a mixture of NiSO4 · xH2O and nickel oxide spinel compound, similar 

in composition to NiFe2O4. The lack of sulfidic Ni emissions from these particular boilers was 

contrary to EPA’s risk assessment assumption that the Ni compound mixture emitted from oil-
INTRODUCTION

1-3

fired utilities is 50% as carcinogenic as Ni3S2 [14]. The Ni cancer risk from residual oil-fired 

boilers may be greatly overestimated [30, 31].

In this investigation, three complementary methods—XAFS, XRD, and water-soluble Ni 

extraction—were used to identify and quantify the Ni species occurring in fly ashes stacksampled in duplicate from three residual oil-fired units, owned and operated by Hawaiian 

Electric Company, Inc. (HECO).The XAFS technique is especially well suited for determining 

Ni speciation because it can directly and nondestructively analyze fly ash filter samples with 

ppm sensitivity [29–34]. XAFS spectroscopy involves recording the variation of the x-ray 

absorption coefficient as a function of energy in the vicinity of a characteristic absorption edge 

for an element of interest. The x-ray flux is supplied from a very intense synchrotron x-ray 

source such as those used in this investigation at the National Synchrotron Light Source (NSLS), 

Brookhaven National Laboratory, New York, and the Stanford Synchrotron Radiation 

Laboratory (SSRL), Stanford University, California. Analysis of x-ray absorption coefficient 

variations in the x-ray absorption near-edge structure (XANES) and extended x-ray absorption 

fine structure (EXAFS) spectral regions of the XAFS spectrum provides structural information 

concerning the local environment of the analyte element in terms of oxidation state, interatomic 

distances, and the type and number of coordinating ligands.

2-1

2

BACKGROUND

XAFS Spectroscopy

XAFS spectroscopy is a powerful method for determining the speciation of an element in any 

material (gas, liquid, crystalline solid, amorphous solid). As its name implies, XAFS 

spectroscopy measures the fine structure associated with one of the characteristic x-ray 

absorption edges of an element of interest. The fine structure is usually of two types (Figure 2-1): 

a fine structure associated with the absorption edge itself and a second fine structure consisting 

of a weak, periodic oscillatory structure that occurs above the absorption edge and normally 

diminishes in intensity the further it is from the edge. Either or both of these regions of the 

XAFS spectrum can be used to determine how the element occurs in a material.

Figure 2-1

XAFS Spectrum of Elemental Selenium Showing the Division of the Spectrum into 

Separate XANES and EXAFS Regions and the Subsequent Generation of a Radial 

Structure Function (RSF) by Fourier Transform of the EXAFS Region of the Spectrum. The 

Peak in the RSF Spectrum Represents X-Ray Scattering Interactions Between the 

Selenium Atom and Its Immediate Neighbors and Can Be Considered a One-Dimensional 

Representation of the Structure Local to the Selenium Atom
BACKGROUND

2-2

The structure associated with the edge itself is known as the XANES region, whereas the 

periodic structure above the edge is known as the EXAFS region. Analysis of each region is 

performed independently. Typically, the XAFS spectrum is first normalized to the edge step and 

then divided into separate XANES and EXAFS regions. The latter region can be further 

manipulated mathematically to obtain a RSF. Where the EXAFS region provides structural 

information, the XANES region provides bonding and valence state information.

Power Plant Sites

The oil-fired power plants that were sampled for Ni speciation are described in Table 2-1. 

AirKinetics, Inc., was at the Honolulu, Kahe, and Waiau power plants April 12–15; June 7–11, 

13, and 14; and May 9–13, 2010, respectively, to perform EPA Section 114 HAP emissions and 

Ni speciation testing. The average electric generation attained for each unit during metals testing 

is presented in Table 2-1.

Table 2-1

Power Plant Descriptions and Abbreviations

Facility Unit Abbreviation Unit Gross Electric Generation, MWe

Honolulu Power Plant (HPP) 9 H9 53

Kahe Power Plant (KPP) 1 K1 85

Waiau Power Plant (WPP) 6 W6 52
3-1

3

EXPERIMENTAL

Residual Oil Analyses

Proximate, ultimate, and metal analyses were performed on composite residual oil samples 

collected from HPP, KPP, and WPP as described in AirKinetics reports [35–37].

Fly Ash Stack Sampling and Analysis Methods

ROFA was sampled isokinetically and in duplicate by AirKinetics personnel for ≤ 5 hr/day from 

the stacks of the three units (Table 2-1) using a modified EPA Method 17 sampling train 

assembly, with the nozzle, probe, and quartz thimble filter maintained at >290°C, well above the 

sulfuric acid dew point temperature. The nozzle and filter holder were constructed of quartz and 

glass, respectively, to prevent contamination of ROFA by metal surfaces. The sampling train and 

procedures used in obtaining representative fly ash samples are described in detail by EPA [38].

The sampling approach employed is consistent with previous investigations of the Ni speciation 

of stack emissions from oil-fired utility boilers and sewage sludge incinerators [26–28, 30, 31,

39]. Plant operating and monitoring data and continuous emission monitoring results indicated 

that the modified EPA Method 17 stack sampling occurred during steady, representative power 

plant operations. In addition to modified EPA Method 17, ROFA samples were sampled 

according to EPA Method 29 and analyzed for Ni as described in Air Kinetics reports [35–37].

All of the quartz thimble samples received by the Energy & Environmental Research Center 

(EERC) and University of Kentucky for analysis are described in Tables 3-1 and 3-2. Several of 

the field and trip blanks were not analyzed using XAFS, but were prepared using microwaveassisted acid digestion (EPA Method 3050 and ASTM International Standard Practice D5513) 

and analyzed using ICP–AES (EPA Method 6010A and 7000 series).

Soluble Ni Extraction

The ROFA filters were exposed to deionized H2O for about 30 minutes. ROFA H2O extraction 

residues were dried in an oven at 50°C before analyzing them using XAFS spectroscopy and 

XRD. The leaching procedure was intended to dissolve readily soluble species such as 

NiSO4·6H2O from the filters and preconcentrate insoluble Ni species such as Ni oxides and 

sulfides. Analyses of ROFA H2O extraction residues minimize potential x-ray absorption and 

diffraction spectral overlaps caused by the presence of multiple Ni species. The extraction 

residue samples are listed in Table 3-2.
EXPERIMENTAL

3-2

Table 3-1

List of Samples Cross-Referenced to XAFS File Names

Power 

Plant and 

Unit

XAFS Analysis 

Facility Thimble Filter Identification XAFS File

HPP 9 SSRL, May 2010 1 H9-Ni-1 NiASH_011.003

2 H9-Ni-2 NiASH_010.003

Field blank H9-Ni-FB NiASH_017.001

Trip blank H9-Ni-TB NiASH_018.001

WPP 6 NSLS, August 

2010

1 W6-Ni-1 NiROFA_025 – 027

2 W6-Ni-2 NiROFA_028 – 030

Field blank W6-Ni-FB NA

Trip blank

1

W6-Ni-TB NA

KPP 1 NSLS, August 

2010

1 K1-Ni-1 NiROFA_031 – 033

2 K1-Ni-2 NiROFA_034 – 036

Field blank K1-Ni-FB NA

Trip blank K1-Ni-TB NA

1 Not analyzed by XAFS.

Table 3-2

List of ROFA H2

Power 

Plant and 

Unit

O Extraction Residue Samples Cross-Referenced to XAFS File Names

XAFS Analysis 

Facility Thimble Filter Identification XAFS File

HPP 9 SSRL, May 2010 2 H9-Ni-2R NiLEACH_026.016

WPP 6 NSLS, August 2010 1 W6-Ni-1R NiROFAL_045 – 048

2 W6-Ni-2R NiROFAL_049 – 053

KPP 1 NSLS, August 2010 1 K1-Ni-1R NiROFAL_037 – 040

2 K1-Ni-2R NiROFAL_041 – 044

Ni XAFS Spectroscopy

Nickel XAFS measurements were made in both absorption and fluorescence geometry at 

Beamline 4-1 at SSRL or at Beamline X-18B at NSLS using a fluorescence detector to detect the 

x-rays emitted by Ni in the investigated materials in response to the x-ray absorption process.

Soller slits and a 6-μm cobalt filter were also used at SSRL to enhance the fluorescent 
EXPERIMENTAL

3-3

signal/noise ratio. Harmonic rejection was accomplished by detuning the intensity of the incident 

beam by about 30%. The spectra were collected as a function of energy from about 200 eV 

below the K absorption edge of Ni at 8333 eV to as much as 1000 eV (16 k) above by measuring 

the absorption and fluorescence signals as a function of the angular rotation of the silicon (220) 

monochromator at SSRL or the Si (111) monochromator at NSLS. The absorption spectrum of a 

Ni0 foil, obtained simultaneously in absorption geometry, was used as the primary energy 

calibration standard in both sets of experiments.

If necessary, the thimble filters were cut in half lengthwise to expose the deposited material, and 

then a 5-cm-long section containing the thickest and most coherent deposit was cut from one-half 

of the filter (Figure 3-1). This section, approximately 3 × 5 cm in size, was taped to the sample 

holder so as to expose the ROFA to an x-ray beam. Photographs of the other filters from KPP 

and WPP Units 1 and 6, respectively, are presented in Appendix A. Excellent-quality 

fluorescence spectra were obtained at SSRL from the samples prepared in this manner using a 

standard Lytle fluorescence detector. Data of almost as good quality were obtained at NSLS 

using a passivated, implanted, planar silicon (PIPS) detector to measure fluorescence radiation. 

The XAFS spectra of eight standard Ni compounds, Ni3S2, NiS, NiS2, NiSO4, NiSO4·H2O, 

NiSO4·6H2O, NiSO4·7H2O, and NiO, obtained during the course of a similar testing program for 

an additional utility provider, were also used as reference spectra for this investigation.

Figure 3-1

Photograph of Deposits on Quartz Thimble Filters from HPP 9. Thimbles Have Been Cut to 

Expose ROFA to the X-Ray Beam
EXPERIMENTAL

3-4

XRD

Representative sections of the ROFA-containing thimble filters were mounted in the XRD for 

analysis. XRD patterns were collected over 5–60° 2-theta with a Bruker D8 Advance theta–

thetax-ray diffractometer system operating at 40 kV and 40 mA (parallel-beam geometry using 

an incident beam Goebel mirror to monochromatize and collimate Cu K-á radiation, 0.02° 2-

theta steps, 7 to 15 sec/step). Diffraction peaks were identified using the Bruker EVA evaluation 

software utilizing the International Centre for Diffraction Data (ICDD) PDF-2 inorganic and 

organic powder diffraction database.
4-1

4

RESULTS AND DISCUSSION

Residual Oil Analyses

Presented in Table 4-1 are proximate and ultimate analysis results for the composite residual oil 

samples. Sulfur only ranged from 0.3 to 0.4 wt%. The calorific (i.e., heating) value of the 

residual oil sampled from WPP Unit 6 is significantly greater relative to the two other oils. 

AirKinetics also had the composite oil samples analyzed for Ni and many other elements, but the 

Ni concentrations were all reported as <5.0 mg/kg [35–37].

Table 4-1

Residual Oil Proximate and Ultimate Analysis Results, as-received basis

Analysis Parameters HPP Unit 9 KPP Unit 1 WPP Unit 6

Avg. ±95% C.I. n 1

Avg. ±95% 

C.I. 2 n Avg. ±95% C.I. n

Proximate Analysis

Water, vol% <0.05 4 <0.05 7 <0.05 5

Ash, wt% <0.01 4 0.02 ±0.01 7 0.05 ±0.02 5

Ultimate Analysis, wt%

Hydrogen 10.9 ±0.1 4 11.6 ±0.2 7 11.1 ±0.6 5

Carbon 87.3 ±0.1 4 87.5 ±0.2 7 84.9 ±4.6 5

Nitrogen 0.25 ±0.07 4 0.21 ±0.08 7 0.31 ±0.07 5

Sulfur 0.37 ±0.01 4 0.39 ±0.02 7 0.32 ±0.00 5

Oxygen 1.23 ±0.22 4 0.38 ±0.19 7 0.74 ±0.14 4

Calorific Value, Btu/lb 17958 ±22 4 17963 ±183 7 18346 ±583 5

1

Average ±95% confidence interval.

2

ROFA Ni Analyses

Number of samples analyzed.

EPA Method 29

Compared in Table 4-2 are the average Ni contents of the ROFAs based on triplicate EPA 

Method 29 results. ROFA from the KPP contains significantly less Ni relative to the other two 
RESULTS AND DISCUSSION

4-2

Table 4-2

Average ROFA Ni Concentrations and Emission Rates

Facility Average Ni ±95% Confidence Interval, wt% Average Ni Emission, lb/106

HPP, Unit 9

Btu

4.26 ±0.97 6.33E-04 ±1.6E-05

KPP, Unit 1 2.40 ±0.37 1.19E-03 ±2.3E-04

WPP, Unit 6 4.78 ±0.14 4.28E-04 ±1.7E-05

power plants, but the Ni emissions from KPP were the highest, probably because it was 

consuming the most oil to produce the most power, as indicated in Table 2-1. Even though 

ROFA from WPP had the highest Ni concentration, its Ni emission rate was intermediate to HPP 

and KPP, probably because it was producing a similar amount of power (Table 2-1) to HPP but 

with a residual oil whose calorific value was significantly higher.

Ni XAFS Spectroscopy

Spectra collected for the HPP Unit 9 samples are shown in Figure 4-1. Spectra of the duplicate 

ROFA samples are very similar, both in overall intensity and appearance. Also compared in 

Figure 4-1 are the corresponding spectra of the trip and field filter blanks and the ROFA H2O 

Figure 4-1

Ni XAFS Spectra Collected at SSRL for Thimble Filter Samples from HPP Unit 9. The 

Spectra for the Field and Trip Blanks Do Not Differ Significantly and, Thus, Superimpose 

as a Single Trace at the Plotted Scale. Inset Shows a Vertical Expansion of the Scale for 

the H9-Ni-2R and Blank Samples
RESULTS AND DISCUSSION

4-3

extraction residue. The spectrum for the residue sample is much weaker relative to bulk ROFA 

samples, indicating that the leaching extracted a large amount of soluble Ni species. No 

significant feature appears at the Ni absorption edge in the blank sample spectra, indicating that 

Ni was below the XAFS detection limit.

Presented in Figure 4-2 are the XAFS spectra for the ROFA samples obtained from KPP and 

WPP Units 1 and 6, respectively. Spectra for the duplicate ROFA samples are essentially 

identical for a given power plant. The field and trip blank thimble filters for the KPP and WPP 

Units 1 and 6, respectively, were not analyzed because ICP–AES measurements, presented in 

Table 4-3, indicated that Ni was not present in high enough concentrations to produce Ni XAFS 

spectra from them. Comparative data for the ROFA H2O extraction residue, H9-Ni-2R, are also 

shown in Figure 4-2.

The Ni XAFS data obtained from the ROFA samples were divided into separate XANES and 

EXAFS regions according to the procedures described in Figure 2-1. Plots of the XANES and 

EXAFS data were prepared for each set of samples from the three units. These spectra are shown 

in Figures 4-3 and 4-4 for the samples measured at SSRL and NSLS, respectively. Spectra 

obtained from the ROFA H2O extraction residue sample, H9-Ni-2R, are also included in 

Figure 4-3.

The Ni XANES and EXAFS spectra of the four ROFA H2O extraction residues from KPP and 

WPP are presented in Figure 4-5. Except for the W6-Ni-2R sample, there was sufficient Ni

Figure 4-2

Ni XAFS Spectra Collected at NSLS for Thimble Filter Samples from KPP and WPP Units 1 

and 6, Respectively. The Spectrum of a ROFA H2O Extraction Residue, W6-Ni-2R, is 

Shown for Comparison
RESULTS AND DISCUSSION

4-4

Table 4-3

Ni Contents of the Filter Blank Samples from KPP and WPP Units 1 and 6, respectively

Sample Ni, ppm

KPP Unit 1 Trip Blank 1.16

KPP Unit 1 Field Blank 0.81

WPP Unit 6 Trip Blank 0.88

WPP Unit 6 Field Blank 1.10

remaining on the filters after the aqueous leaching procedure to produce acceptable-quality 

spectra at all stages of the spectral data reduction process. For the W6-Ni-2R sample, however, 

the EXAFS region was too noisy to permit a meaningful RSF spectrum to be generated from the 

EXAFS (k3chi) spectrum.

Nickel XAFS spectra of various reference Ni compounds are presented in Figures 4-6–4-8.

Figure 4-6 compares the Ni XAFS data obtained for three different nickel sulfates, NiSO4

(anhydrous), NiSO4·H2O, and NiSO4·6H2O. Figure 4-7 shows the processed XAFS spectra for 

the three Ni sulfides, Ni3S2, NiS, and NiS2. These spectra differ significantly from those shown 

in Figure 4-6 for the Ni sulfates in three major ways: (i) there is no sharp large peak at the 

absorption edge for the Ni sulfides compared to the Ni sulfates; (ii) the major peak in the RSF 

occurs at about 2.0 Å for the Ni sulfides and at about 1.5 Å for the Ni sulfates; and (iii) the peak 

maximum in the derivative XANES spectra occurs at a significantly lower energy for the Ni 

sulfides than for the Ni sulfates (compare Figures 4-6 [b] and 4-7[b]). These differences make it 

easy to distinguish Ni sulfides from Ni sulfates. Furthermore, the Ni sulfides all show significant 

absorption in a small region of the spectrum (8335 to 8340 eV) that is devoid of significant 

absorption for Ni oxide and sulfate compounds.

Figure 4-8 shows Ni XAFS data for three Ni oxide species, NiO, NiFe2O4, and NiAl2O4. These 

spectra are the best that we have in our database for these compounds; however, it seems 

possible to improve on the quality of these data. In particular, the spectral data for the nickel 

ferrite, NiFe2O4, are quite weak, as indicated by the noisy nature of the derivative XANES 

spectrum (Figure 4-8 [b]). The EXAFS regions and RSF spectra of all three compounds (Figures 

4-8 [c], [d]) are complex, which is typical of the high-symmetry structures exhibited by 

compounds with cubic crystal structures, MO (halite [NaCl]-type) and AB2O4 (spinel-type). For 

NiO, peaks representing the 4th and 5th and even more distant coordination shells are 

discernable in the RSF (Figure 4-8 [d]). For both NiO and NiFe2O4, the second peak in the RSF 

spectrum at ≈ 2.5–2.6 Å exceeds the height of the NiO peak at ≈ 1.5 Å. This peak derives from 

the second nearest coordination shell in the oxide structure and consists of metal (Ni or Fe) 

atoms. The enhanced back-scattering power of the higher-atomic-number elements, Ni and Fe, 

compared to oxygen, as well as the larger coordination number for the second coordination shell 

in these structures, account for the enhanced nature of this peak relative to the first coordination 

shell peak.
RESULTS AND DISCUSSION

4-5

Figure 4-3

Ni XAFS Spectra of Duplicate ROFA Samples and a ROFA Residue Sample from the Stack of HPP Unit 9; (a) XANES Spectra, 

(b) Derivative XANES Spectra, (c) EXAFS (K3

chi) Spectra, and (d) RSF Spectra. Data Collected at SSRL
RESULTS AND DISCUSSION

4-6

Figure 4-4

Ni XAFS Spectra of Duplicate ROFA Samples from the Stacks of KPP and WPP Units 1 and 6, Respectively; (a) XANES Spectra, 

(b) Derivative XANES Spectra, (c) EXAFS (K3

chi) Spectra, and (d) RSF Spectra. Data Collected at NSLS
RESULTS AND DISCUSSION

4-7

Figure 4-5

Ni XAFS Spectra of ROFA Residues Corresponding to KPP and WPP Units 1 and 6, Respectively; (a) XANES Spectra, 

(b) Derivative XANES Spectra, (c) EXAFS (K3chi) Spectra, and (d) RSF Spectra. Data Collected at NSLS
RESULTS AND DISCUSSION

4-8

Figure 4-6

Ni XAFS Spectra of NiSO4, NiSO4·H2O, and NiSO4·6H2O; (a) XANES Spectra, (b) Derivative XANES Spectra, (c) EXAFS (K3

chi) 

Spectra, and (d) RSF Spectra
RESULTS AND DISCUSSION

4-9

Figure 4-7

Ni XAFS Spectra of Ni3S2, NiS, and NiS2; (a) XANES Spectra, (b) Derivative XANES Spectra, (c) EXAFS (K3

chi) Spectra, and 

(d) RSF Spectra
RESULTS AND DISCUSSION

4-10

Figure 4-8

Ni XAFS Spectra of NiO, NiFe2O4, and NiAl2O4; (a) XANES Spectra, (b) Derivative XANES Spectra, (c) EXAFS (K3

chi) Spectra, and 

(d) RSF Spectra
RESULTS AND DISCUSSION

4-11

All of the RSFs for the ROFA samples (Figures 4-3 [d] and 4-4 [d]) have a small but significant 

peak at 2.5 Å, which is always more prominent for the corresponding ROFA H2O extraction 

residue samples (Figure 4-5 [d]). Of the sulfates, only NiSO4 shows a significant second 

coordination shell peak (Figure 4-6 [d]); however, the peak is located at ≈ 2.8 Å, rather than 

2.5 Å. Conversely, NiSO4·6H2O, which is most likely the Ni sulfate contributing to the spectra 

of the residual oil samples, lacks a significant second coordination shell peak. Also, none of the 

Ni sulfide species exhibit a peak at 2.5 Å in their RSFs (Figure 4-7 [d]). These observations 

suggest that the source of the 2.5-Å peak in the ROFA sample spectra is a minor amount of 

NiFe2O4 or NiO rather than a Ni sulfate or sulfide compound. For the ROFA H2O extraction 

residues, the enhanced nature of the 2.5-Å peak, especially for the H9-Ni-2R sample (Figure 4-

3[d]) and K1-Ni-1R and-2R samples (Figure 4-5[d]), indicates the prevalence of NiFe2O4

XANES Pre-Edge Systematics

or 

NiO.

Based on the spectral features for the Ni sulfate and sulfide standards (Figures 4-6 and 4-7), there 

are several systematic differences that enable a distinction between Ni sulfate and sulfide 

species. Oxidic Ni species, such as NiO and various Ni spinels (NiFe2O4, NiAl2O4, NiV2O4, 

etc.), are similar to Ni sulfates because the Ni2+ ion is bonded to oxygen anions, thus their XAFS 

spectra have more in common with those of Ni sulfates than Ni sulfides. Indeed, the spectra for 

NiFe2O4 and NiAl2O4

Detailed examinations of the pre-edge region (8330–8340 eV) of the XANES and derivative 

XANES spectra for all Ni

show much the same pre-edge characteristics as the Ni sulfate standards. 

These systematic differences were used to evaluate whether a sulfidic Ni species was present in 

ROFA.

2+ compounds revealed that there is a ubiquitous peak or inflection 

feature at about 8332–8333 eV that occurs because of the electron transition from Ni 1s orbitals 

to 3d orbitals induced by the x-ray absorption process. Furthermore, the intensity of this feature 

can be used to discriminate Ni2+

As indicated in Figure 4-9, the relative height of the inflection feature is always more than 0.05 

for metallic Ni and Ni sulfides and less than 0.025 for Ni sulfate and oxide phases, except for 

NiO, which exhibits an intermediate value of about 0.032. Figure 4-10 presents the normalized 

pre-edge inflection feature heights determined from the derivative XANES spectra of the bulk 

ROFA samples and their corresponding ROFA H

ions coordinated with sulfur anions versus oxygen anions. 

Plotted in Figure 4-9 are the relative intensities of the inflection feature for various Ni 

compounds.

2O extraction residues. The relative heights for 

the bulk ROFAs and corresponding residues range similarly between 0.009 and 0.017, whereas 

in comparison, the range for a suite of ROFA samples from Florida power plants was much 

smaller at 0.015 to 0.018. Clearly, data for the ROFA samples measured at NSLS (KPP and WPP 

Units 1 and 6, respectively) have values consistently lower than those measured at SSRL for 

ROFA samples from HPP Unit 9. This difference is attributable to resolution differences 

between the two synchrotrons, principally as a result of different monochromator crystal 

orientations. The use of the Si (111) crystals at NSLS results in a somewhat broadened and less 

intense pre-edge peak at NSLS compared to that measured at SSRL.
RESULTS AND DISCUSSION

4-12

Figure 4-9

Comparison of the Normalized Height of the Pre-Edge Inflection Feature at 8333 eV in Ni 

Derivative XANES Spectra for Ni Compounds

Figure 4-10

Comparison of the Normalized Height of the Pre-Edge Inflection Feature at 8333 eV in 

Derivative Xanes Spectra of Rofa Sampled from the Three Hawaiian Units. Light Bars 

Represent Data for the ROFA H2O Extraction Residue Samples
RESULTS AND DISCUSSION

4-13

Since the inflection feature height values for the ROFA samples from Hawaii are less than or 

comparable to those for the Ni sulfates and oxides, such small values indicate that there may be a 

minimal contribution from Ni sulfides. However, the difference between the two synchrotrons in 

the intensity of this feature implies that we cannot develop the height of the pre-edge peak as a 

general quantitative measure for estimating sulfide contents; such correlations would have to be 

restricted to a given monochromator orientation or beamline.

In a companion study of ROFA emissions from Florida power plants, various correlations were 

established between composition and absorption intensity at the positions of the major peaks in 

the Ni derivative XANES spectra for mixtures of Ni3S2 and NiSO4·6H2O, Ni3S2 and NiFe2O4, 

and NiS and NiSO4·6H2O. From these correlations, the following equations were derived for 

estimating the proportions of Ni3S2

For Ni

or NiS in ROFA:

3S2–NiSO4·6H2O mixtures: Ni3S2

For Ni

, % = 813 h(8336) – 4.3 [Eq. 1]

3S2–NiFe2O4 mixtures: Ni3S2

For NiS–NiSO

, % = 892 h(8336) – 13.8 [Eq. 2]

4·6H2

Equations 1 and 3 are applicable to the bulk ROFA samples in which NiSO

O mixtures: NiS, % = 952 h(8337) – 8.0 [Eq. 3]

4·6H2

Using Equations 1–3, Ni

O is the major 

phase, while Equation 2 is more appropriate for the Ni extraction residues after Ni sulfates have 

been removed.

3S2 and NiS contents were estimated from the derivative Ni XANES 

spectra; estimates are presented in Table 4-4. The proportions in Table 4-4 are ≤ 3 % of the total 

Ni present in the bulk ROFA, but the analytical uncertainty is ±2% so the values are essentially 

equal to zero. These proportions are maximum estimates for Ni3S2 and NiS in the bulk ROFA 

samples because NiSO4·6H2O has the lowest h (8336) value of all the standards. Substitution of 

any other Ni sulfate or a Ni oxide species for NiSO4·6H2O in the calibration procedure results in 

smaller estimates of the Ni sulfide proportions, as would the presence of a third component such 

as NiFe2O4 or NiO in the calibration procedure. Applying similar calculations to the ROFA H2O 

extraction residue samples, produces slightly higher estimates for Ni3S2 and NiS; however, for 

the ROFA H2O extraction residues, it is likely that NiFe2O4 or NiO are present and should be 

used in the calibration procedures rather than a Ni sulfate. Using NiFe2O4 in the calibration 

procedure actually results in negative Ni3S2 estimates for most of the ROFA H2O extraction 

residues, based on the use of h (8336) values in Table 4-4 and Equation 3. Such negative values 

probably indicate that some Ni sulfate remained on the ROFA H2

In conclusion, a detailed examination of Ni pre-edge XANES systematics indicates that the 

Ni

O extraction residues.

3S2 or NiS contents of ROFA from the three HECO units are negligible and comprise ≤ 3% of 

the total Ni, if any. This simple two-component calibration procedure is sufficient because both 

Ni sulfides are negligible in all the ROFA samples. A more sophisticated three-component 

approach would be applicable if significant amounts of either or both of the sulfides were 

present.
RESULTS AND DISCUSSION

4-14

Table 4-4

Estimates of NiS and Ni3S2 in ROFA from Three HECO Units

Sample

Bulk ROFA ROFA H2

h(8336)

O Extraction Residues

Ni3S2 h(8337) , 

% NiS, % h(8336) Ni3S2 h(8337) , % NiS, %

H9-Ni-1 0.0043 -1 0.0067 -2 NA NA 1 NA NA

H9-Ni-2 0.0042 -1 0.0065 -2 0.0079 2 (-7) 0.0145 2 6

W6-Ni-1 0.0054 0 0.0103 2 0.0114 5 (-4) 0.0175 9

W6-Ni-2 0.0058 0 0.0091 1 0.0172 10 (2) 0.0072 -1

K1-Ni-1 0.0072 2 0.0113 3 0.0126 6 (-3) 0.0171 8

K1-Ni-2 0.0079 2 0.0108 2 0.0139 7 (-1) 0.0154 7

Least-Squares Fitting of Ni XANES and EXAFS Spectra

In addition to the Ni derivative XANES spectral correlations (Equations 1–3), a least-squares 

fitting of Ni XANES and EXAFS spectra was performed to evaluate the Ni species contents of 

the ROFAs. The most reliable simulation approach consisted of combining the spectra for the 

bulk ROFA (consisting predominantly of NiSO4·6H2O) and corresponding ROFA H2O 

extraction residue (consisting mainly of NiFe2O4 and/or NiO) with that of NiSO4·6H2O and 

NiO. These simulations were performed for the EXAFS (k3

chi) spectra of all six samples. As 

indicated in Table 4-5, these simulations confirm that NiSO4·6H2O is the predominant sulfate 

phase. Some of the fits were marginally improved with the addition of NiO in the least-squares 

fitting, especially for the W6-Ni-1 and -2 samples. However, we suspect that NiO is a major 

component of the Ni remaining in the ROFA H2O extraction residues and that the difference in 

least-squares fitting between fits with and without NiO is largely an experimental artifice. This is 

most likely the case for the K1-Ni-1 and -2 samples, where the Ni proportion as NiO is relatively 

minor and clearly complements the Ni % for the ROFA H2

Least-squares fitting of bulk ROFA and corresponding ROFA H

O extraction residue sample in the 

fitting.

2O extraction residue Ni EXAFS 

spectra to linear combinations of spectra for two or more standard compounds (NiSO4·6H2O, 

NiFe2O4

The least-squares fitting results indicate that the Ni in H9-Ni-1 and -2 samples are predominantly 

(>95%) present as NiSO

, and/or NiO) was performed as shown in Figure 4-11 and reported in Table 4-6. The 

fits did not require the presence of Ni sulfide(s). For the H9-Ni-1 and -2 samples, several 

different fits were statistically similar, and these results are also included in Table 4-6, in 

addition to the best linear combination fit.

4·6H2O, with only very minor amounts of NiFe2O4 and/or NiO. In a

corresponding ROFA H2O extraction residue, H9-Ni-2R, NiSO4·6H2O remains dominant 

despite the large reduction in soluble Ni as indicated by the reduction in the overall absorption 

intensity
RESULTS AND DISCUSSION

4-15

Table 4-5

Results of Least-Squares Fitting of Combined Bulk ROFA and Corresponding ROFA H2O 

Extraction Residue Spectra to Spectra for NiSO4·6H2

Sample

O and NiO

EXAFS Region (3–10 Å-1

Residue, %

) Fitting

NiSO4, % NiO, % Red χ

H9-Ni-1

2

11 89 – 0.124

H9-Ni-2 5 95 – 0.159

W6-Ni-1 9 77 14 0.407

W6-Ni-2 3 84 14 0.452

K1-Ni-1

28 72 – 0.320

18 76 6 0.269

K1-Ni-2

26 74 – 0.372

15 78 7 0.302

Figure 4-11

Examples of Least-Squares Fits to EXAFS Spectra of ROFA Samples: Bulk ROFA Sample, 

K1-Ni-2 (left), and ROFA H2O Extraction Residue, K1-Ni-1R (right). The Spectra Are Fitted 

to a Linear Combination of EXAFS Spectra for NiSO4·6H2O, NiFe2O4

(Figure 3-1). Each of the two Ni oxides comprise about 20% of the total Ni in H9-Ni-2R. The 

proportions of oxidic Ni in the bulk ROFA samples from the other two units were significantly 

larger than those in the H9-Ni-1 and -2 samples. Least-squares fitting indicated that between 

10% and 20% of the Ni in these unleached samples was present as NiFe

, and NiO

2O4 and/or NiO. In the 

K1-Ni-R1 and -R2 samples, NiSO4·6H2O is reduced to about 30%, while NiFe2O4 and NiO 

account for the remaining Ni. Spectra of the ROFA H2O extraction residue W6-Ni-1R and -2R 

samples were too weak to perform similar analyses.
RESULTS AND DISCUSSION

4-16

Table 4-6

Results of Least-Squares Fitting of EXAFS Spectra (3–10 Å-1) for Bulk ROFA and ROFA 

H2O Extraction Residues to Linear Combinations of the Corresponding Spectra for 

NiSO4·6H2O, NiFe2O4

Sample

, and NiO

Total Ni, %

Red. χ

NiSO

2

4·6H2O, % NiFe2O4, % NiO, %

H9-Ni-1

96 4 – 0.177

97 – 3 0.169

96 2 2 0.165

H9-Ni-2

100 – – 0.180

98 2 – 0.168

98 – 1 0.173

98 2 0 0.169

H9-Ni-2R 63 16 21 0.600

W6-Ni-1 85 – 15 0.471

W6-Ni-2 91 – 9 0.485

K1-Ni-1 83 10 7 0.438

K1-Ni-2 81 9 9 0.314

K1-Ni-1R 34 45 21 0.821

K1-Ni-2R 34 39 27 1.720

Analyses of XAFS spectra obtained for ROFA samples from Hawaiian oil-burning power plants 

indicate that NiSO4·6H2O is the most abundant Ni-bearing phase in them. Minor amounts of 

oxide phases such as NiFe2O4

XRD Analyses

and NiO are also present, but in subordinate amounts. No 

evidence exists for the presence of Ni sulfide(s) or other Ni sulfates.

XRD analyses were performed on the bulk ROFA samples primarily to identify crystalline 

phases. X-ray diffractograms for the six ROFA samples are presented in Appendix B. The XRD 

results in Table 4-7 for the bulk ROFA samples indicate that amorphous components, glass and 

unburned carbon, are ubiquitous in the ROFAs. NiSO4·6H2O and CaSO4 are also present in all 

the ROFAs. The KPP Unit 1 ROFA is distinguished by the presence of FeNi2O4 which is very 

similar to the NiFe2O4 compound identified by XAFS. XRD analysis results of the ROFA H2O 

extraction residues indicated that the H2O extraction removed most of the ROFA, and no 

crystalline phases were detected. The sensitivity (i.e., limit of detection) of XRD is much poorer 

relative to XAFS spectroscopy.
RESULTS AND DISCUSSION

4-17

Table 4-7

Bulk ROFA Phase Assemblages

ROFA 

Sample

NiSO4·6H2O FeNi2O4 CaSO4 Glass and Unburned Carbon

H9-Ni-1 X X X

H9-Ni-2 X X X

K1-Ni-1 X X X X

K1-Ni-2 X X X X

W6-Ni-1 X X X

W6-Ni-2 X X X

1

An “X” denotes that the corresponding amorphous or crystalline phase was identified using XRD.

5-1

5

CONCLUSIONS

The Ni speciation of ROFA samples collected in duplicate on quartz thimble filters from three 

oil-burning utility boilers was evaluated using two x-ray techniques, XAFS spectroscopy and 

XRD. ROFA deionized H2O extraction residues were also analyzed for Ni speciation using 

XAFS and XRD. Ni pre-edge XANES systematics and the least-squares fitting of XANES and 

EXAFS spectra of the six ROFA samples with those from standard Ni compounds indicated that 

the major Ni phase, comprising 80%–95% of the total Ni, was NiSO4·6H2O with subordinate 

proportions, ≤ 20% of the total Ni, present as a spinel compound, similar in composition to 

NiFe2O4 and/or NiO. XRD confirmed the presence of NiSO4·6H2O and NiFe2O4 but not NiO. 

Spectral evidence for the presence of Ni sulfide compounds (Ni3S2, NiS, or NiS2

Table 5-1

Ni Species Emission Estimates

) was absent 

and, at most, does not exceed 3% of the Ni in any sample. Based on these species proportions 

and the total Ni emission rates in Table 4-2, the emission rates for the Ni species were calculated 

as indicated in Table 5-1.

Facility NiSO4·6H2O, lb/106 Btu NiFe2O4 ± NiO, lb/106 Btu NixSy, lb/106

HPP, Unit 9

Btu

5.06E-4─6.01E-4 ≤ 1.27E-4 ≤ 1.90E-5

KPP, Unit 1 9.52E-4─1.13E-3 ≤ 2.38E-4 ≤ 3.57E-5

WPP, Unit 6 3.42E-4─4.07E-4 ≤ 8.56E-5 ≤ 1.28E-5

6-1

6

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A-1

A

APPENDIX

PHOTOGRAPHS OF QUARTZ THIMBLE FILTERS

Figure A-1

Photograph of a Thimble Filter with ROFA Sampled from KPP Unit 1
Appendix

A-2

Figure A-2

Photograph of a Duplicate Thimble Filter with ROFA Sampled from KPP Unit 1

Figure A-3

Photograph of a Thimble Filter with ROFA Sampled from WPP Unit 6
Appendix

A-3

Figure A-4

Photograph of a Duplicate Thimble Filter with ROFA Sampled from WPP Unit 6

B-1

B

APPENDIX

X-RAY DIFFRACTOGRAMS OF BULK ROFA AND Ni EXTRACTION 

RESIDUES

Figure B-1

X-ray Diffractogram of ROFA Sample from HPP Unit 9 Indicating Spectral Matches with 

NiSO4·6H2O and CaSO4
Appendix

B-2

Figure B-2

X-ray Diffractogram of Duplicate ROFA Sample from HPP Unit 9 Indicating Spectral 

Matches with NiSO4·6H2O and CaSO4
Appendix

B-3

Figure B-3

X-ray Diffractogram of ROFA Sample from KPP Unit 1 Indicating Spectral Matches with 

NiSO4·6H2O, NiFe2O4, and CaSO4
Appendix

B-4

Figure B-4

X-ray Diffractogram of Duplicate ROFA Sample from KPP Unit 1 Indicating Spectral 

Matches with NiSO4·6H2O, NiFe2O4, and CaSO4
Appendix

B-5

Figure B-5

X-ray Diffractogram of ROFA Sample from WPP Unit 6 Indicating Spectral Matches with 

NiSO4·6H2O and CaSO4
Appendix

B-6

Figure B-6

X-ray Diffractogram of Duplicate ROFA Sample from WPP Unit 6 Indicating Spectral 

Matches with NiSO4·6H2O and CaSO4