  SEQ CHAPTER \h \r 1 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL EXPOSURE RESEARCH LABORATORY

Human Exposure and Atmospheric Sciences Division

Research Triangle Park, NC 27711

REPORT FROM THE ELECTRON MICROSCOPY LABORATORY

DATE: 	9/7/06

PROJECT:	Arsenic in Paint Flaked from CCA-Treated and Untreated Wood

ANALYSIS:	Scanning Electron Microscopy/Energy-Dispersive X-ray
Spectrometry	

ANALYST:	Robert Willis

Sample Description

Two samples of paint flakes were provided for SEM analysis: sample 10U
(from untreated wood) and sample 9T (from CCA-treated wood). Pieces of
each sample were mounted flat on 13mm tacky carbon tabs affixed to
standard aluminum SEM stubs. Both the exterior faces (side exposed to
the air) and the interior faces (in contact with the wood) were examined
by SEM. In addition, pieces of each sample were mounted vertically in
order to examine cross-sections of both samples. All samples were coated
with ~ 200 Å of carbon to minimize sample charging during the SEM
analysis.

SEM Analysis

Samples were manually examined using a LEO/Zeiss Model 440 SEM equipped
with a PGT IMIX energy-dispersive X-ray spectrometer (EDX) for providing
elemental composition information on individual particles. (EDX does not
allow for determining oxidation states of metals.) Operating parameters
were: 20 keV electron beam energy and 0.3 nA beam current. Particle
images were collected using both a secondary electron (SE) detector and
a backscattered electron (BSE) detector.

Results

Individual arsenic-bearing particles (or particle inclusions) on the
order of a micron in size were found on both surfaces of sample 9T using
the BSE detector. This detector responds to the effective atomic number
of the feature being analyzed, so that particles of high atomic number
appear much brighter than those of lower atomic number. Arsenic-bearing
particles show up bright against the lower atomic number paint matrix
(see Figure 1) which is dominated by titanium dioxide and aluminum
silicate. The presence of arsenic was confirmed by the EDX spectra
(Figures 2, 4, 5). Almost all arsenic-bearing particles showed a similar
composition characterized by high Fe, possibly Ti, and small amounts of
Cr, Cu, and As. (Since TiO2 was ubiquitous in the paint, it is possible
that the Ti observed in the EDX spectra of the As-rich particles was
essentially “matrix background” contributed by neighboring or
underlying paint particles.) The source of the iron which was always
appeared in association with the arsenic is unknown, but it is known
that iron oxide has an affinity for binding to arsenic (Shibata et al.,
2006 and references therein). 

Although As-rich particles were found on both the exterior and interior
surfaces of 9T samples, my qualitative observation is that more
particles were found on the interior surface. A more quantitative
analysis would be needed to confirm this however. The interior surfaces
of the paint flakes which were in contact with the CCA-treated wood
showed numerous wood fibers attached to the paint. Some of the
arsenic-bearing particles appeared in proximity to wood fibers and may
be physically associated with the wood fibers (Figure 3, top) while
other arsenic particles were not found physically near wood fibers,
including all the arsenic-rich particles found on the exterior surfaces
(Figure 3, bottom).  

Arsenic was not detected in the “bulk matrix” of sample 9T, i.e.,
collecting x-ray spectra at random locations on the paint flake did not
yield detectable arsenic signals. One cannot conclude, however, that
arsenic is not present throughout the sample. If it is present, then its
bulk concentration is below the EDX detection limit (~ 1 wt %). Also,
arsenic-rich particles smaller than about 100 nm may have gone
undetected at the magnifications used in this analysis. 

No arsenic-bearing particles were observed in the 10U (untreated)
samples.

References

Shibata, T.; Solo-Gabriele, H.; Dubey, B.; Tonwsend, T.; Jacobi, G.
Arsenic leaching from mulch made from recycled construction and
demolition wood and impacts of iron-oxide colorants. Environ. Sci.
Technol. 2006, 40, 5102-5107. 



Figure 1. Top: Low mag BSE image of sample 9T showing bright As-bearing
particle in center. Bottom: The same particle at higher magnification
imaged with SE detector. Surrounding features are TiO2-rich associated
with paint. Scale bar indicates that particle size is ~ 2 µm.

Figure 2. Top: EDX spectrum of As-bearing particle in Figure 1. Spectrum
is dominated by Fe, Si, Ti, and O. Bottom: expanded spectrum showing Cr,
Cu and As. Arsenic has x-ray peaks at 10.5 and 11.7 keV. Arsenic was
always associated with Cr, Cu and high Fe.



Figure 3. Top: As-rich particle located on wood fiber (bright feature
running NW to SE through image) on interior surface of sample 9T.
Bottom: As-rich particle on exterior surface of 9T surrounded by
sub-micron particles of paint pigment.

Figure 4. Image and spectrum of As-bearing particle in sample 9T.



Figure 5. Image and spectrum of As-bearing particle in sample 9T



Figure 6. Top: As-bearing agglomeration in sample 9T. Bottom: Lower-mag
BSE image showing a line of bright As-bearing particles running slightly
downhill from left to right across the center of the image. All of the
bright, angular features are As-bearing.

