United States
Environmental Protection
Agency
Research and Development
Atmospheric Research and Exposure
Assessment laboratory
Research Triangle Park, NC 27711
EPA/600/S3-91/052 Feb. 1992
«rEPA Project Summary
Laboratory and Field
Evaluations of a Methodology for
Determining Hexavalent
Chromium Emissions from
Stationary Sources
Anna C. Carver
Development of methodology for
sampling and analysis of chromium to
support stationary source regulations
was initiated in 1984. This study was
initiated to determine whether chro-
mium emissions should be regulated
under Section 112 of the Clean Air Act
National Emissions Standards for Haz-
ardous Air Pollutants (NESHAP). To
support stationary source regulations,
it is important that (1) the sampling
procedure not change the chromium
valence state during sampling and (2)
an analytical technique for measuring
low concentration levels of chromium
be available. These goals are achieved
with the current EPA "Draft Method for
Sampling and Analysis of Hexavalent
Chromium at Stationary Sources.*
The draft method utilizes a recirc-
ulating system to flush impinger re-
agent into the sampling nozzle during
sample collection. Immediate contact
of the stack gas with impinger reagent
"fixes" the chromium valence state. .
Ion chromatography coupled with a
post column reaction system and an
ultraviolet visible detector is used to
analyze Cr(VI) in the parts per trillion
range.
Field tests were conducted at metal
plating facilities, industrial cooling
towers, municipal waste incinerators,
sewage sludge incinerators, and haz-
ardous waste incinerators. It was at
the hazardous waste facility that the
new method was proven to have ac-
ceptable precision and essentially no
conversion in the sample train. Stan-
dard deviations for the sampling runs
were determined.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Sampling and Analytical
Procedures
A "dry" test method was developed for
collection of Cr(VI) with intended applica-
tion at stationary sources with filterable,
dry emissions. This method involved col-
lection of particulate emissions by use of
Method 5 (Appendix B, 40 CFR Part 60).
Under Method 5, stack gas is isokinetically
drawn through a sampling probe and glass
fiber filter. Colorimetric analysis using a
spectrophotometer with and optimum
wavelength of 540 nm was used to detect
the specific wavelength generated by
diphenylcarbazide when complexed with
Cr(VI) in the filter digest. Tests employing
this method were conducted at a
ferrochrome smelter, a chemical plant, and
a refractory brick plant.
Testing of the Method 5 sampling train
continued in various studies at coal fired
boilers, municipal waste combustors and
chrome plating facilities. It was determined
that this method was unable to effectively
collect chromium from these sources, thus
an impinger technique (MethodlS-type
train) was developed to collect the mist or
droplets containing soluble chromium.
?§g> Printed on Recycled Paper
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Twelve paired test runs were performed
at a chromium plating facility to compare
the two test methods.
The impinger train consisted of a stain-
less steel nozzle, a heated glass lined
probe, fitter bypass, two reagent impingers
containing 0.1 N sodium hydroxide
(NaOH), an empty impinger, filter, and
silica gel impinger. Further studies and
data comparisons allowed the elimination
of the filter. The train was operated
Isokinetfcally using Method 5 sampling
procedures. Sample was recovered using
a deionized water rinse. Analysis for Cr(VI)
was the same as that described for the
Method 5 sampling technique. The
impinger method sampling train corrected
some of the problems associated with
chromium determinations at facilities with
effluent streams containing mist or aero-
sols. Documented results from chrome
plating facilities indicated that the method
was acceptable. Questions remained,
however, concerning Cr(VI) conversion.
Further field studies of the stationary
sources with potential Cr(VI) emissions
included a series of cooling towers. It was
at cooling tower facilities that the need for
a low level analytical technique for mea-
surement of hexavalent chromium was
critical. Methods of reducing sample vol-
ume for better detectability with the colori-
metric method proved to be unacceptable.
Also, reducing sample volume increases
the chances of chemically altering the va-
lence form of the collected chromium be-
fore Cr(VI) determinations have been
achieved.
A second series of cooling tower test
incorporated the following changes to the
sampling technique: (1) the collection me-
dia was changed from deionized water to
0.1 N NaOH, and (2) an additional rinse
of the sampling train with nitric acid was
added to the recovery process. These
rinses were included to determine how
much chromium.remained in the impingers
after the first rinse, and to loosen any
insoluble chromium from the walls of the
impingers.
A very important aspect of this series of
cooling tower tests was the realization that
chromium is converting somewhere be-
tween the tower and the sampling train.
Efforts to determine where the conversion
was occurring and to identify an analytical
technique with acceptable detection limits
were undertaken in a series of laboratory
and field tests.
The analytical technique selected for
evaluation was ion chromatography (1C)
coupled with a Cr(VI)-specific post column
reaction. The Cr(VI)-specific post column
reaction employs the diphenylcarbazide
chromophor identical to the colorimetric
procedures used in the earlier tests. 1C
however provides much greater sensitiv-
ity. A preconcentration technique extends
the sensitivity of this system to the parts
per trillion range. This 1C procedure al-
lows separation of Cr(Vl) in the basic col-
lection media, eliminating potential biases
from sample preparation.
The development of this analytical tech-
nique revolutionized the development of a
stationary source test method for mea-
surement of Cr(VI). It was now possible
to directly measure Cr(VI) at levels as low
as 100 parts per trillion.
The development of a sampling train
which would prevent the conversion of
Cr(VI) was necessary to obtain represen-
tative measurements of stack gas. Be-
cause laboratory studies showed that the
potential exist for conversion of Cr(VI) to
Cr(lll) in the glass-lined probe of the
Method 13-type train, a sampling device
which eliminated the "dry" probe surface
area was designed. Employing this de-
sign allows immediate contact of the Cr(VI)
with the basic impinger reagent as the
stack gas enters the sampling train. This
immediate contact allows the reagent to
dissolve the chromium and stabilize the
Cr(VI) in the gas stream. The recirculating
reagent also provides continuous rinsing
of the sample line. To minimize back-
ground contamination often noted with the
glass components of the impinger train,
the new recirculating (RC) train was con-
structed of Teflon. Conventional glass
impingers were used for the nitric acid
and silica gel impingers. A schematic of
this recirculating sample train is presented
in Figure 1.
Following the development of the RC
sampling train, laboratory studies were
conducted to select the most suitable col-
lection reagent. Several series of labora-
tory and field studies were conducted. To
trace the chromium during the experimen-
tal procedures, a radioactively labeled
chromium isotope, (51Cr(VI)) was em-
ployed. This radioisotope has a half-life
of 27.7 days, which provides ample analy-
sis time yet relatively fast disposal. The
isotope is also available with high specific
activity (high radioactivity) from a minute
concentration of Cr(VI) which is not de-
tectable by the ion chromatographic tech-
nique.
Employing this radioisotope, the first
series of laboratory studies was conducted
and based on the results of the study,
isopropyl alcohol/sodium hydroxide (I PA/
NaOH) solution was selected for field
evaluation.
Following field tests of the IPA/NaOH
reagent and analytical difficulties associ-
ated with analyzing the samples collected
in this reagent, a second series of test
were conducted to evaluate other reagents.
In this series of test, the NaOH reagent
was found to be acceptable for preventing
conversion of Cr(VI).
A third laboratory experiment was con-
ducted when it was discovered that the
effects of dissolved gases, which can bring
about the reduction of Cr(VI), could be
removed by purging the sample train with
nitrogen following the test run.
The newly design RC sampling train
does not include a filter. Paniculate mat-
ter is collected in the impinger portion of
the sampling train during sample collec-
tion. The presence of this particulate mat-
ter can generate a positive or negative
bias in Cr(VI) determinations. Because of
the potential conversion an immediate
post-run filtration was conducted.
Finally, field evaluations to compare the
new RC sampling train with the previous
impinger train were conducted. Two field
tests were performed at a cooling tower
facility. In both tests, the RC train and the
Method 13-type train were operated si-
multaneously.
During the initial field test, four collo-
cated trains (two RC and two Method 13-
type) were operated at two separate loca-
tions. This test was conducted to com-
pare the conversion percentages for each
type of train.
The second field test was designed to
determine the extent of conversion of
Cr(VI) to Cr(lll) in the RC train. The ra-
dioactive tracer, 51Cr(VI), was added to the
impinger reagent prior to sampling. The
isotope would be exposed to the same
conditions as the native Cr(VI) collected
from the cooling tower. If any 51Cr(VI)
converted to Cr(lll) during sampling then
the train would most likely be responsible
for the conversion. j
In conjunction with efforts to develop
procedures for sampling and analysis of
chromium and nickel species from sewage
sludge incinerators, another field study was
conducted. Two types of sewage sludge
incinerators were visited, a multiple-hearth
facility and a fluidized bed facility. Chro-
mium determinations were difficult at these
facilities due to the high organic content
of the effluent stream.
The final study was an emissions field
test performed at a hazardous waste in-
cinerator. The facility included a rotary
kiln, secondary combustion chamber,
electrostatic preciprtator and packed col-
umn scrubber. The procedures followed
were those of the current "Draft Method
for Determination of Cr(VI) at Stationary
Sources."
For developmental purposes, the
impinger reagent was again spiked with
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the radioisotope 51Cr(VI). The radioisotope
data was used to determine recovery effi-
ciency of the method and the amount of
conversion that occurred in the sampling
train.
Results and Discussion
The original hexavalent chromium
method was limited to stationary sources
with filterable dry emissions. Also, the limit
of detection for the colorimetric technique
was determined to be 0.1 ppmv, but the
concentration of the samples had to be
above 10 ppmv due to a series of dilu-
tions required during sample preparation.
When the test method was evaluated at
the three facilities mentioned earlier, it was
assumed that trivalent and hexavalent
chromium were collected on the.filter and
in the probe rinse in the same ratio as
emitted from the stack. Later testing re-
vealed that collection procedures can
cause changes in the valence state of the
chromium compound.
To compare the new Method 13-type
sampling procedures with the Method 5
sampling procedures 12 paired test runs
were performed at two separate locations
of a chrome plating facility. The data
indicated that the dry filter method re-
sulted in values 10 to 25 percent lower
than the Cr(VI) values obtained from the
impinger trains. At this time, the Method
5-type sampling procedures for collecting
chromium were deemed unacceptable.
The Method 13 evaluations at chromium
plating facilities indicated that, in almost
all cases, the amount of Cr(VI) collected
appeared to be over 80 percent of the
total catch. Trivalent chromium however,
was determined as the difference between
total chromium and Cr(VI). Based on
these results the Method 13-type sam-
pling techniques were acceptable for the
determination of chromium. This type of
sampling train corrected some of the
problems associated with chromium de-
terminations at facilities with effluent
streams containing mist or aerosols. The
question of chromium conversion re-
mained.
The first set of Method 13-type data
obtained at a cooling tower resulted in
highly variable data. The restrictions of
the analytical detection limits coupled with
the use of deionized water as the collec-
tion media rather than a basic solution
may have contributed to the variable re-
sults. Possible conversion in the sampling
train during sampling may have also added
to biased results.
At the facility hosting the second series
of cooling tower tests, as much as 90
percent of the chromium collected was
trivalent. The chromium in the recirculating
cooling water at the facility was typically
99 percent Cr(VI). These data lead to the
conclusion that conversion of Cr(VI) to
Cr(lll) was occurring in the cooling tower,
in the Method 13-type sampling train, or
in both. This conversion phenomenon
and the inability to directly analyzed for
Cr(VI) in low concentration levels were
possibly the key factors in the Cr(VI)
method development studies.
Finally, an analytical method was iden-
tified for direct low-level Cr(VI) analyses.
The limit of detection for measurement of
Cr(VI) using an ion chromatographic post
column reaction (IC/PCR) system was de-
termined to be 0.16 parts per billion (ppb),
with a quantifiable limit of 0.5 ppb. When
preconcentration was employed, the limit
of detection was lowered to 0.05 ppb.
The upper range of detection without dilu-
tion of the samples was determined to be
1 ppm.
The IC/PCR was linear over two orders
of magnitude! Precision, in terms of rela-
tive standard deviation, was 5.2 percent
based on analysis of 10 samples. Accu-
racy, determined by analysis of a suitably
diluted EPA audit sample averaged 6.7
percent for 7 analyses over a one month
period. With the addition of an insitu
preconcentration technique it was deter-
mined that up to 30 mL of a 0.5 ppb
standard solution could be loaded on the
preconcentration column before significant
breakthrough occurred. With the injection
of a 20 ml sample, the sensitivity of the
technique was increased over 60-fold,
yielding a limit of quantification of less
than 100 ppt. Accuracy at this level, in
terms of percent error, was determined to
be ± 9.5 percent.
Laboratory studies conducted to select
the most suitable collection reagent yielded
data which indicated that an IPA/NaOH
solution would be the most effective col-
lection media. However, the anion ex-
change column used in sample analysis
was not designed for use with organic
compounds and the capacity of the col-
umn was decreased over the course of
the sample analyses. Also, the IPA was
evaporating at extremely rapid rates dur-
ing field sampling, thus resulting in loss of
impinger solution before the end of the
sample run.
In the second series of tests, NaOH
was found to be acceptable. In two sepa-
rate test runs, recovery of Cr(VI) was
above 97 percent.
The third laboratory test conducted in-
cluded the use of a posttest nitrogen purge.
Sodium hydroxide and sodium bicarbon-
ate reagents had favorable results with
over 90 percent Cr(VI) recovery. As shown
in Table 1, these data indicate that the
trains purged with nitrogen have a higher
final pH than those not purged. The purge
helps to reduce dissolved gases in the
collected sample which may lead to oxi-
dation of the Cr(VI) ion.
Results from the initial field test of the
new RC train (compared with the Method
13-type train) indicated that conversion
was still occurring in either the cooling
tower or the sample trains. The RC train,
however, did show an average conver-
sion 20 percent less than that of the
Method 13-type train.
The data collected from the second field
evaluation of the RC train revealed Cr(VI)
to Cr(lll) conversions of 71 percent for the
RC train and 81 percent for the Method
13-type train. The probe of the Method
13-type train was recovered separately
and revealed conversions of 88 percent.
(Conversion of Cr(VI) to Cr(lll) was deter-
mined by ratios of Cr(VI) to total chro-
mium.) Following the cooling tower tests,
all further studies were conducted only
using the RC sampling system.
Results of testing conducted at the
sewage sludge incinerator indicated that
this method may not be acceptable at
facilities with high organic content. Con-
version of the chromium was as high as
45 percent at one of the facilities. Analy-
sis of inlet sample was impossible due to
loss of column capacity.
The final field study, at a hazardous
waste incinerator, was the first field test to
indicate essentially no sample bias from
the sampling procedures. The radioactive
data indicated that 100 percent of the
radioactive spike was accounted for when
all components of the train were collected.
The impinger solution however, only ac-
counted for 75 percent of the total.
Analysis of the filter digestion solutions
and 1C eluant indicated that only 1 per-
cent of the Cr(VI) spiked into the train was
reduced during sampling.
Hexavalent chromium and total chro-
mium determinations were made for each
sampling train, and means and standard
deviations were calculated for each run.
The results are shown in Table 2.
The variance of the Cr(VI) measure-
ments were tested using the Bartlett test
and was found to be homogeneous
throughout the entire six runs. Therefore,
it was possible to calculate standard devia-
tions from the combined variances that
were more representative of the data than
the standard deviation of any run. Thus,
the standard deviations of the Cr(VI) mea-
surements for Runs 1-3 and 4-6 were
0.9 and 9.5 u.g/dscm, respectively. The
value for the total chromium measurements
was 27 ng/dscm.
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Ttbto 1, Media Collodion Experiment Using Nitrogen Purge
Solution
Initial
Posttest
(without purge)
Posttest
(with purge)
Ammonium acetate
pH Level
Average (% )Conversion
Sodium Bicarbonate
pH Level
Average (%)Conversion
Sodium hydroxide
pH Level
Average (%)Conversion
Sodium acetate
pH Level
Average (%)Conversion
13
5.5
-22.12
8.0
6.59
8.0
16.71
5.5
100
6.00
-10.49
9.00
1.00
9.00
9.70
6.00
100
Test results from this field evaluation
Indicated that the RC train performed ad-
equately. The collection efficiency of the
Impingers was improved from 75 percent
to 95+ percent with the addition of a fourth
deionized water rinse.
Conclusions and
Recommendations
The major goals of the methods devel-
opment program were (1) to develop the
sampling procedures that do not change
the oxidation state of chromium during
sampling and (2) to identify a readily
available analytical technique for measur-
ing low-concentration levels of Cr(VI).
These goals have been achieved.
Many years of challenging work have
resulted in a draft test method for chro-
mium that does not affect the valence
state of the compound as rt enters the
sample train, and the identification of an
analytical technique with the ability to
analyze concentrations of Cr(VI) in the
ppt range.
Several major discoveries were made
during the development of the draft
method:
It was determined that conversion of
Cr(Vl) to Cr(lll) was occurring in the
sample collection system. Until this
sample collection bias was elimi-
nated, no accurate data could be
obtained.
When an investigation of a surro-
gate compound for chromium was
evaluated, it was discovered that
emissions ratios and cooling tower
water ratios were not the same. This
assumption had been applied to
chromium for several earlier cool-
ing tower tests. Calculating a value
for Cr(VI) was no longer possible
based on these assumptions. A di-
rect analysis technique was neces-
sary.
Poor precision between sampling
trains during the test evaluations
generated questions about the re-
covery process. It had been as-
sumed that the collected chromium
was water soluble. In truth, the vari-
ability between sampling runs was
caused in part by the poor recovery
of insoluble chromium. An acid rinse
was added to remove the insoluble
chromium from the glassware.
A recirculating sample train was de-
veloped to "rinse" the sample probe
continuously during sample collec-
tion. This design allowed immediate
contact of the stack gas with the
impinger reagents.
A low-level analytical technique for
analyzing Cr(VI) was utilized. Ion
chromatography with a post column
reaction (IC/PCR) allowed detection
of Cr(VI) in the ppt range. The avail-
ability of this technique increased
the variety of sources to which the
sampling method could be applied.
The use of a radioactive tracer
greatly increased knowledge of the
behavior of chromium in the sam-
pling train. For the first time, the
chromium could be "followed"
through the collection procedure.
Use of the tracer ultimately allowed
accurate determination of conversion
caused by sampling.
A study of the effects of sulfur diox-
ide (SO2) on Cr(VI) was conducted.
This study generated information
which (1) led to better choices of
collection media and (2) emphasized
the need to remove SO2from the
collected sample.
A nitrogen purge following sample
collection was added to the method.
The nitrogen purge removed dis-
solved gases that may react with
chromium. The solutions were then
recovered and filtered, which re-
moved insoluble Cr(NI) compounds
that may be slowly oxidized.
Many of the discoveries mentioned
above may have invalidated data that had
been collected prior to correction of the
sampling or analytical problems. Thus ex-
cept for data obtained using the draft
method, any data should be regarded with
caution.
Further studies to optimize and confirm
the validity of this method should be con-
ducted:
Because the analytical technique (IC/
PCR) of this draft method is not
commonly available to most labora-
tories, alternative techniques would
be desirable. It is possible that a
preconcentration technique, coupled
with colorimetric spectrophotometric
analysis, could be an alternative to
IC/PCR, although no studies have
been conducted to date.
Comparisons of a glass recirculating
sample train with the Teflon RC train
should be conducted. Some data has
been generated to indicate that glass
components are not acceptable when
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Table 2. Measurement of Cr(VI) from Hazardous Waste Incinerator
Condition 1. Aqueous Cr+3 fed into process.
Condition 2. Aqueous Cr+6 fed into process.
Isokinetic Moisture Total Total
Sampling Content Cr+6 Chromium
Sample Rate fag/dscm) (\ig/dscm)
Run Train (%) (%)
1 A 97.9 19.2 0.357 116.259
B 100.3 21.8 0.415 187.124
C 95.4 21.7 0.367 153.084
D 100.1 21.9 0.179 110.081
Average 21.2 0.330 141.637
RSD 6.1% 31.4% 25.3%
2 A No results
B No results
C 98.8 23.0 0.305 138.288
D 98.2 21.7 0.355 87.611
Average 22.4 0.330 112.950
RSD 3.4% 8.8% 25.9%
3 A 99.7 23.2 0.410 42.196
B 95.2 23.3 0.440 47.199
C 93.3 22.7 0.575 48.737
D No results
Average 23.1 0.475 46.044
RSD 1.4% 17.5% 7.0%
^^^\
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Cr+6/
Total
Chromium
0.3%
0.2%
0.2%
0.2%
0.2%
24.7%
0.2%
0.4%
0.3%
35.5%
1.0%
0.9%
1.2%
1.1%
11.4%
Isokinetic Moisture Total Total Cr+6/
Sampling Content Cr+6 Chromium Total
Sample Rate fag/dscm) fog/dscm) Chromium
Run Train (%) (%)
4 A 105.7 21.8 89.828 114.150 78.7%
B 102.8 22.5 107.569 132.319 81.3%
C 99.2 23.3 106.129 126.789 83.7%
D 103.6 23.4 120.440 127.646 94.4%
Average 22.8 105.991 125.226 84.5%
RSD 3.5% 11.8% 6.2% 8.1%
5 A 104.6 24.1 76.618 117.440 65.2%
B 106.0 25.4 76.666 00.136 76.6%
C 102.6 24.4 74.534 160.521 46.4%
D 103.8 23.7 76.729 99.606 77.0%
Average 24.4 76.137 119.426 64.2%
RSD 3.0% 1.4% 24.0% 22.3%
6 A 102.8 20.6 43.426 56.117 77.4%
B 91.1 24.7 57.280 129.892 44.1%
C 95.0 24.0 50.419 59.788 84.3%
D 92.5 24.1 52.977 72.414 73.2%
Average 23.4 51.025 79.553 71.8%
RSD 8.0% 11.4% 43.1% 24.7%
Glass Impinger __
Teflon Impingers (7~~}\ I
p u nr i r f i f . — -
nl 75m
'OH 0.1NK(
Wi
IT! I IlP
f *'"" 75 ml * Empty Silica
OH 0.1NKOH Gel
tier and Ice Bath
5-type
Meterbox
J
I
(
Figure 1. Schematic of recirculating sampling system.
quantifying Cr(V\); however, glass
coupled with recirculation has not
yet been investigated.
Laboratory studies should be con-
ducted to document that C-flex tub-
ing does not cause oxidation of
Cr(VI). Because this material is not
inert, it is possible that off-gassing
during heating releases compounds
that could convert the chromium.
And evaluations are needed if the
draft method is to be applied at lo-
cations with temperatures above
400°F and with high SO2 concentra-
tions.
•&V.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40161
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Anna C. Carver is with Entropy Environmentalists, Inc. Huntington Beach, CA 92649
Joseph E Knoll is the EPA Project Officer (see below).
The complete report, entitled "Laboratory and Field Evaluations of a Methodology for
DeterminingHexavalent Chromium Emissions from Stationary Sources, "(Order No. PB92-
101336AS; Cost: $26.00 (subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Off her can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S3-91/052
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