Leaching from CCA-Treated Wood into Soils: Preliminary
PIXE Studies
1
R.F.Kelly, 1I.I. Kravchenko, 1J.C. Kuharik, 1H. A. Van Rinsvelt, 1F. E.Dunnam,
2
J.Huffman
1
Department of Physics, University of Florida, 2School of Forest Resources and Conservations, University of
Florida
Abstract. Widespread use of chromated copper arsenate (CCA) as a wood preservative has led to increasing public
concern regarding possible toxic contamination of areas surrounding CCA-treated structures, e.g., decks, playground
equipment, etc. Appreciable leaching of arsenic, chromium, and copper into soils adjacent to such structures has been
demonstrated via standard techniques of analytical chemistry. The advantages of PIXE [rapid analysis, quick sample
turnover, possible lower cost] suggest its application to this area of interest. PIXE studies in our laboratory of CCAcontaminated soil samples show good agreement with previous analyses of As, Cu, Cr, and other heavy-elemental
content, with some variability in diffusion rates.
chemistry in sample preparation. Characteristic Xrays, produced by bombardment of a sample with
energetic (1 - 3 MeV) protons, yield unambiguous
determination of the chemical elements from which
they are emitted. Average analysis time of
approximately one hour per sample (after preparation)
can be shortened via batch processing. PIXE results
are herewith briefly described and compared with
more conventional analyses of soil samples taken from
the vicinity of selected CCA-treated wood structures.
INTRODUCTION
The public has become increasingly concerned with
the environmental impact of toxic materials leached
from wood products treated with chromated copper
arsenate (CCA) as a preservative. As a result,
children's playground equipment and other treated
wooden structures are being razed, sometimes with
few or no tests to determine the extent of any
contamination that may have occurred. That leaching
does take place however is well-documented in
numerous publications [1]. Many follow-up
investigations are under way; a local study of leaching
from treated posts that have been in place for several
decades is currently in progress [2].
Testing methods for the quantitative determination
of soil contamination by CCA leachate include
inductively coupled atomic emission spectrometry (ICAES), atomic absorption and Zeeman effect thermal
spectroscopy, and others. Many of these require
considerable sample preparation time including
extensive wet chemistry, thus delaying availability of
results.
Particle Induced X-ray Emission (PIXE)
analysis affords rapid results from quite small (< 1 g)
samples and eliminates the need of much wet
METHOD
Soil samples of approximately 10 grams each were
collected with an aluminum trowel at various distances
and depths from the wooden structures. 100 milligram
portions were selected from each sample and dried
overnight at 85 C. Specimens of approximately 20 mg
were then taken from each portion and distributed
uniformly in a 1 cm spot on a 2.5 cm-diameter
aluminized Mylar backing and fixed in position with a
1% solution of polystyrene in benzene. After drying
overnight the samples were bombarded with 2.5 MeV
protons from the NEC electrostatic accelerator in the
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
428
University
of
Florida
Physics
Department.
Characteristic X-rays were detected with a Kevex
Si(Li) detector and the resulting spectra were analyzed
with the code Pixfit [3].
RESULTS
Figure 1 displays a typical X-ray spectrum from a
surface sample of undisturbed soil taken adjacent to
FIGURE 1. X-ray spectrum from CCA leachate in soil top level
the base of a CCA-treated post, in place for
approximately 15 years (figure 2). X-ray intensity
peaks indicating the presence of As, Cu, Fe, and other
elements are clearly visible. The prominent peaks due
to arsenic are indicative of the method's sensitivity.
DISCUSSION
Preliminary results presented in Table 1 demonstrate a
good comparison between PIXE and standard
chemical analysis. This report describes a feasibility
study to compare PIXE with conventional analytical
procedures for soil leachates; hence no detailed results
for other elements are presented. However other recent
quantitative comparison studies of lacustrine
sediments (based on the AGV-1 USGS geological
standard [4] and employing yttrium as an internal
spectrum calibrator) have further validated PIXE as a
viable method for elemental analysis
of soil
components [5]. The data reported in Table 2 indicate
that in this particular instance the elements of interest
migrate only a foot or so from the treated object due to
FIGURE 2. This is CCA treated posts in place for
approximately 15 years, forest test grounds at the University
of Florida
429
TABLE 1. Comparison of Arsenic in Soil [PPM], Commercial Analysis [CA] vs. PIXE
Sample Location
Arsenic, PPM [CA]
Arsenic, PPM [PIXE]
1
12
10
2
2.5
3
(bremsstrahlung).
On the other hand, detection
efficiency improves with increasing atomic number
becoming very good for the region (20< Z < 80) which
includes not only arsenic and copper but the majority
of heavy metals of concern as environmental
contaminants. Small samples (a few grams), rapid
sample preparation and analysis with little or no wet
chemistry and good sensitivity recommend PIXE as a
relatively quick and efficient analysis tool for heavier
elements in soils. Further PIXE studies of CCA
leaching, taking the cited variables (soil type,
climate/rainfall history, etc.) into account would be of
considerable value.
leaching and that arsenic appears to diffuse somewhat
more extensively than copper or chromium. However,
it should be noted that in all of the cases reported
herein the soil was quite sandy and porous;
furthermore, no correlation with local rainfall
variations was attempted. Further investigation of
CCA leaching in soils of differing porosity would
clarify the issue. A disadvantage of PIXE is its
limitation to the detection of chemical elements only
(not compounds); furthermore, its sensitivity declines
significantly for atomic numbers below approximately
Z = 15 (phosphorus) due to diminishing energy of the
characteristic X-rays, coupled with the presence of
unavoidable low-energy background radiation
TABLE 2. Depth concentration distribution of As, Cr and Cu near a CCA treated post.
Element
Control, Top Soil
Layer, PPM
0”-4” Layer,
PPM
8”-12” Layer,
PPM
32”-36” Layer,
PPM
As
0
139
117
13
Cr
14
134
12
4
Cu
10
111
20
2
REFERENCES
1. Stilwell, D. E. and Gorny, K. D., Bull. Environ. Contam.
Toxicol. 58, 22-29 (1997).
2. Huffman, J. B. , and Morrell, J., to be published.
3. Coldwell, R. L., and Van Rinsvelt, H. A., "Pixfit - A
Special Analysis Program for PIXE" in Application of
Accelerators in Research and Industry -1997, edited by
J.L.Duggan and I.L.Morgan, AIP Conference
Proceedings 475, New York: American Institute of
Physics, 1997, pp.555-558.
4. Carlsson, L-E., Akselsson, K. R., Nucl. Instrum. Meth.
181, 531-537 (1981).
5. Kravchenko, I. I., Kuharik, J., Kelly, R. F., Dunnam, F.
E., Van Rinsvelt, H. A., to be published.
430