EPA-650/2-74-124
DECEMBER 1974
Environmental Protection Technology  Series


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                   RESEARCH REPORTING SERIES


Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields.  These series are:

          1.  ENVIRONMENTAL HEALTH EFFECTS RESEARCH
          2 .  ENVIRONMENTAL PROTECTION TECHNOLOGY

          3.  ECOLOGICAL RESEARCH
          4.  ENVIRONMENTAL MONITORING
          5.  SOCIOECONOMIC ENVIRONMENTAL STUDIES

          6.  SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS

          9.  MISCELLANEOUS

This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series.  This series describes research performed to
develop and demonstrate instrumentation,  equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution.  This work provides the new or improved
technology  required for the control  and treatment of pollution sources
to meet environmental quality standards.

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                                       EPA-650/2-74-124
               DEVELOPMENT
         OF  SAMPLING  METHOD
FOR  TOTAL  ATMOSPHERIC  SELENIUM
                       by

           William J. Barrett and Herbert C. Miller

         Engineering and Applied Sciences Department
               Southern Research Institute
                2000 Ninth Avenue South
               Birmingham, Alabama 35205
                Contract No. 68-02-1220
                ROAP No. 26ACX, Task 19
               Program Element No. 1AA010
            EPA Project Officer: Eva Wittgenstein

             Chemistry and Physics Laboratory
           National Environmental Research Center
         Research Triangle Park, North Carolina 27711
                    Prepared for

          OFFICE OF RESEARCH AND DEVELOPMENT
         U.S. ENVIRONMENTAL PROTECTION AGENCY
               WASHINGTON, D.C.  20460

                    December 1974

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                        EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development.
EPA,  and approved for publication.  Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
                                  11

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                           PREFACE


Experimental work on this project began in July 1973 and was
completed in June 1974.

M. Patricia Alewine, Associate Chemist, and Michael Rooks,
Assistant Chemist, assisted in performing the work described
in this report.

Discussions with members of the staff of the National
Environmental Research Center at Research Triangle Park and
with Dr. William Fulkerson and his associates at Oak Ridge
National Laboratory were helpful in planning this work and
in interpreting the results.
                             111

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                      TABLE OF CONTENTS


                                                         Page

Preface                                                   iii

List of Tables                                            vi



Sections

I       Introduction                                       1

II      Summary                                            3

III     Conclusions                                        4

IV      Recommendations                                    6

V       Methods for the Determination of Selenium          8

VI      Laboratory Generation of Selenium Vapors and      14
          Aerosols

VII     Laboratory Evaluation of Methods of Sampling      21
          for Selenium in Air

VIII    Selenium Sampling of the Ambient Atmosphere       28
          and Investigation of the Chemical Forms of
          Atmospheric Selenium

IX      Conclusions Concerning the Nature of Atmospheric  35
          Selenium

        References                                        38

        Technical Report Data (EPA Form 2220-1)           42
                              v

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                       LIST OF TABLES

                                                         Pag
No.                                                      	2

1       Recovery of Selenium from Filters with Analysis
          by the Methylene Blue Method

2       Nebulization-Generator Output                     •*-

3       Selenium Analyses of Generator Effluent at 74 °C   17

4       Selenium Analyses of Generator Effluent at 27°C   18

5       Particle-Size Distribution of Selenium Aerosol    19
          Generator Effluent

6       Selenium Retention of Treated Filters for Sam-    24
          pling the Dry-SeO2 Generator Effluent
          (Preliminary Results)

7       Collection Efficiencies of Bubbler Solutions for  26
          (CH3)2Se Vapor

8       Retention of Selenium from the Dry-Se02 Genera-   27
          tor by Various Bubbler Solutions

9       Selenium Content of Ambient Atmospheric Samples   30

10      Andersen Cascade Impactor Sampling of the         31
          Ambient Atmosphere

11      Effect of Digestion Treatment on the Determina-   33
          tion of Selenium by the DAN Method
                             VI

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                           SECTION  I

                         INTRODUCTION
This  report  describes  the experimentation undertaken and the
results  obtained  in  connection with a project to develop sam-
pling and  analytical methods  for total atmospheric selenium.
The primary  purposes of  this  contract were to develop a col-
lection  technique for  the quantitative collection of total
atmospheric  selenium and to characterize the forms in which
selenium occurs in the atmosphere.  A secondary, but essen-
tial,  requirement was  that an analytical method be selected
and demonstrated  to  be suitable for the routine laboratory
determination  of  the selenium in field-collected samples.

Although little is known about the occurrence, distribution,
and concentration of selenium in the atmosphere, the element
and its  compounds may  be as significant with regard to health
as some  others that  have been more thoroughly investigated.
Selenium compounds are known  to be toxic, although their pre-
cise  role  in human biochemistry is not fully understood.  A
threshold  limit value  (TLV) of 0.2 mg/m3 has been established
for selenium compounds (as Se) by the American Conference of
Governmental Industrial  Hygienists.1  Recognition of the pos-
sibility of  increasing exposure of the populace to selenium
from  combustion and  incineration processes demands that more
attention  be given to  the methodology of sampling and charac-
terization of  atmospheric selenium.

Selenium is  widely distributed in nature and occurs only in
very  small concentrations, most often in association with sul-
fide  minerals.  It is  concentrated from soils in some areas
by certain types  of  plants.   Even in these more concentrated
natural  forms, however,  the proportion of selenium is very low.
The average  concentration of  selenium in the earth's crust
has been estimated to  be only about 0.09 yg/g (parts per
million).

Major  sources of  atmospheric  selenium include nonferrous
metallurgical operations, glass-melting furnaces, the combus-
tion of  fossil fuels,  and the incineration of solid wastes,
especially paper.2'3   It has been estimated that in 1969 the
commercial consumption of selenium in the United States was
660 kkg  (728 tons).  Emissions to the atmosphere were esti-
mated  to total 900 kkg (986 tons); of this amount, 65% was
derived  from the  burning of coal,  21% from glass-manufacturing
processes,  9% from the production of nonferrous metals, and
7% from  the  burning  of fuel oils.1*

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Th^re appears to be no firm information in the literature
the chemical species in which selenium occurs in tne amDJ
atmosphere.  This lack of information may be attri  ^
the difficulties arising from the extremely low total con e
tration levels of selenium and to the fact that the selenium
compounds that may be present can constitute no more tnan
small fraction of the total atmospheric contaminants in any
given sample.

The approach undertaken relative to the sampling problem was
based on the use of retentive filters for the collection of
particulate material containing selenium and evaluations of
chemically treated filters for retention of selenium that may
occur in the gaseous or vapor form.  In the initial phase of
the investigation, laboratory aerosol generators were set up
for evaluations of filter materials, and some effort was
expended in establishing the 2,3-diaminonaphthalene  (DAN)
fluorometric method as the primary analytical technique  for
selenium.  Later, samples of atmospheric particulate material
were analyzed for selenium and procedures were  investigated
for determining the relative amounts of selenium present in
different oxidation states in the particulate material.  Also,
additional selenium aerosol and vapor generators were set up,
characterized, and used in evaluations of other candidate sam-
pling devices.

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                         SECTION II

                           SUMMARY
The primary purposes of this contract were to develop a tech-
nique for the quantitative collection of total atmospheric
selenium and to characterize the forms in which selenium
occurs in the atmosphere.

The approach undertaken relative to the sampling problem was
based on the use of retentive filters for the collection of
particulate material containing selenium and evaluations of
chemically treated filters for the retention of selenium that
may occur in the gaseous or vapor form.  In the initial
stages of the investigation, laboratory aerosol generators
were set up for evaluations of filter materials and, on the
basis of comparison with other methods, the 2,3-diamino-
naphthalene (DAN) fluorometric method was selected as the
primary analytical technique for selenium.  Later, samples of
atmospheric particulate matter were analyzed for selenium and
procedures were investigated for determining the relative
amounts of selenium present in different oxidation states in
the particulate material.  Also, additional selenium aerosol
and vapor generators were set up, characterized, and used in
evaluations of other candidate sampling devices.

It appeared that the selenium present in the ambient atmo-
sphere exists predominantly as, or in association with, par-
ticulate matter, and that little or no loss of selenium by
vaporization occurred during continuous sampling with filters,
Thus, it was tentatively established that conventional partic-
ulate sampling was sufficient for collecting the bulk of atmo-
spheric selenium.  Vapor forms of selenium are believed to
make a negligible contribution to total atmospheric selenium.

The most probable chemical forms in which atmospheric sele-
nium exists were determined to be the element and the dioxide
(or selenite).

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                         SECTION III

                         CONCLUSIONS


On the basis of the experimental results presented in this
report—primarily the results of laboratory experiments with
a dry-selenium dioxide aerosol generator and the analyses of
atmospheric particulate samples—the following conclusions
were reached.

     • Selenium occurs in the atmosphere primarily as
       selenium dioxide and elemental selenium, in
       roughly equal proportions.  Selenium in the +6
       oxidation state does not occur in the atmosphere.
       Efforts to establish the presence or absence of
       organic selenium were not conclusive.

     • Selenium occurs in the atmosphere primarily in
       particulate form or in association with particu-
       late material.  The contribution of vapor forms
       to total atmospheric selenium is believed to be
       negligible.

     • Little or no selenium is lost by vaporization dur-
       ing continuous sampling with filters.

     • Selenium in the atmosphere is not primarily asso-
       ciated with the smaller size fractions, as it has
       been shown to be in stack emissions, but is most
       abundant in the particle sizes near the mass mean
       diameter.

     • Conventional high-volume particulate sampling is
       adequate for the collection of atmospheric sele-
       nium.

     • Of the reagent-impregnated filters evaluated for
       collecting vapor forms of selenium, none were
       demonstrated to be applicable to atmospheric sam-
       pling, and, moreover, they were not shown to be
       effective in laboratory experiments.

     • Analyses of particulate samples collected at an
       urban site of heavy industrial pollution and
       samples collected at a site of only moderate
       pollution suggested that the selenium concentra-
       tions in the air at the two sites were the same —

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  essentially at a uniform background level—and
  that the sources in the heavily polluted area
  were not significant emitters of selenium.

• Errors can occur in the estimation of selenium
  concentrations by the DAN-fluorometric method if
  samples are subjected to a strong oxidative treat-
  ment without a subsequent reduction step to con-
  vert Se(VI) to Se(IV).

• Cyclohexane was found to be superior to toluene
  or hexane as the extracting solvent for a piaz-
  selenole, primarily because of the much lower
  blank fluorescence observed with cyclohexane.

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                         SECTION IV

                       RECOMMENDATIONS


As a result of this investigation, several recommendations^
for future research can be made.  First, additional labora-
tory studies should be conducted to evaluate treated filter
materials for the retention of elemental selenium vapor.
Studies such as these take on added significance in view of
recent reports that selenium emissions from coal-fired power
plants initially exist as elemental selenium vapor.  It is
reported that the sulfur dioxide present in the stack efflu-
ent is sufficient to insure the reduction of any selenium
dioxide to the element.  Also, the importance of these studies
is underscored by the fact that coal burning represents the
single most significant source of atmospheric selenium.  To
facilitate the experimental work, heated selenium metal
(possibly in an inert atmosphere) should serve as a conve-
nient source of the metallic vapor in a laboratory generator.
Some effort should also be expended to answer several ques-
tions concerning the nature of the elemental selenium vapor:
what is the allotropic form of the metal in the vapor phase?;
what is the stability of the metal vapor with respect to
oxidation or reduction in the atmosphere?; and does the
selenium metal vapor tend to condense rapidly to form an
aerosol?  After these questions concerning the laboratory
investigation are satisfactorily answered, sampling for sele-
nium in the vicinity of a coal-fired power plant should be
conducted.

Secondly, additional methods for sampling the volatile organic
selenide—dimethyl selenide—should be considered.  Sorption
on an activated carbon surface with subsequent desorption,
oxidation, and analysis is a potential avenue for future
investigation.  If this volatile selenide could be success-
fully sampled, its possible presence in the ambient atmosphere
could then be investigated.

Finally, additional methods of selenium analysis which may
offer increased sensitivity and specificity should be evalu-
ated in greater depth.  For example, the gas chromatographic
methods briefly studied in this investigation should be
reconsidered.  These methods are based on the gas chromato-
graphic separation of a piazselenole with subsequent detec-
tion by electron capture.  An additional recently described
gas chromatographic method which shows the promise of
increased sensitivity and specificity involves the detection

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of a piazselenole by microwave emission spectrometry (MES).
Because the MES detector response is not a function of the
organic moiety comprising the piazselenole but a function of
the selenium atom itself, the method appears to offer a
significant improvement over conventional electron capture
detection of the piazselenole.  For these reasons the GC-MES
technique should also be considered for selenium determina-
tion in atmospheric samples.

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                          SECTION V

          METHODS FOR THE DETERMINATION OF SELENIUM


Because of the importance of establishing a convenient and
reliable analytical method for selenium, three of the several
methods presented in the literature were evaluated for their
suitablity in the routine analysis of atmospheric samples.
The three methods which appeared to be the most adaptable for
the purposes of this study were the fluorometric procedure
using DAN, a gas chromatographic method using DAN or either
4-chloro-o-phenylenediamine or 4-nitro-o-phenylenediamine,
and a catalytic method based on the reduction of methylene
blue.  Although neutron activation analysis, atomic absorption
spectrophotometry, and spark source mass spectrometry have
seen a considerable amount of use recently for the determina-
tion of elements in atmospheric particulate material, they
were not deemed especially suitable for the purposes of this
project.  However, a new, highly sensitive technique worthy
of future consideration has been developed at the Oak Ridge
National Laboratory.5  This method for the determination of
selenium is based on the gas chromatographic separation of a
piazselenole with microwave emission spectrometric  (MES)
detection.  The method appears to offer even more sensitivity
than the methods considered in this study.
THE METHYLENE BLUE METHOD

The catalytic methylene blue method developed by West is a
simple procedure involving no instrumentation and few
reagents.6  It is useful over a range of 0.1 to 1.0 yg of
selenium.  It has been used by West to determine selenium in
smoke from trash burning,7 and in cigarette papers, other
papers, and tobacco.8

The method is based on the catalytic effect of trace amounts
of selenium in the reduction of methylene blue by sodium sul-
fide.  Ethylenediaminetetraacetic acid (EDTA) is used as a
general masking agent, and formaldehyde,  which suppresses the
reducing power of sodium sulfide, is used to stabilize the
blank.  By taking advantage of the inducing effect of Fe(Ill),
the limit of detection can be made as low as 0.1 yg of sele-
nium.6  Calibration data are obtained by  measuring the time
required for complete decolorization of methylene blue after
mixing the reagents in the presence of known quantities of

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selenium.  A linear calibration curve is obtained by plotting
the reciprocal of  the time  (min~1) versus micrograms over the
range of 0.1 to  1.0 yg of selenium.

In our evaluations of the methylene blue method, typical
results were those obtained in an experiment involving the
recovery of selenium from spiked  glass-fiber filters by
extraction with  water.  These results are presented in
Table 1.  This exercise demonstrated the uncertainty in the
methylene blue method of analysis for amounts of selenium
less than about  0.25 yg.  Below this level, reproducibility
suffered from the  visual uncertainty in the color change at
the end point.   On the other hand, simultaneous determinations
of selenium in identical samples  by the DAN-fluorometric
method resulted  in selenium recoveries in excess of 90% of the
theoretical  (spiked) amount.  Thus, the low recovery of sele-
nium in the case of the methylene blue method suggested that
the filters may  contain a surface material that inhibits the
catalytic effect of selenium on the reduction of methylene
blue by sulfide.   For these reasons, and specifically for the
reason of limited  sensitivity, the methylene blue method was
not selected as  the primary method of analysis for atmospheric
selenium.
      Table  1.  RECOVERY OF SELENIUM FROM FILTERS WITH
             ANALYSIS BY THE METHYLENE BLUE METHOD
Amount of Se
added, yg
0.0
0.05
0.10
0.25
0.50
0.75
1.0
Amount of
recovered,
0.09
0.19
0.19
0.23
0.35
0.52
0.67
Se
yg







THE GAS CHROMATOGRAPHIC METHOD

The gas chromatographic method is relatively new.  It was
introduced by Nakashima and T6ei.9  The compound 4-chloro-p_-
phenylenediamine reacts with selenious acid  (Se(IV)) to form
the 5-chloropiazselenole which is detectable by electron-
capture gas chromatography.  Later, Shimoishi and Toei used
4-nitro-o-phenylenediamine to form 5-nitropiazselenole, which
is more Sensitive to electron-capture than the chloro

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 compound.10   Shimoishi has used the gas               n sea
 method to determine  selenium in pure tellurium   and in sea
 water12.   As  little  as 2 ng of selenium was detected by this
 method.

 More  recently an  article by Young and Christian13 further
 pointed  out the utility of the gas chromatographic method of
 selenium analysis.   In this example, selenium  (IV) reacted
 with  2,3-diaminonaphthalene at pH 2 to form the well
 established piazselenole, which was then extracted witn
 hexane.   An aliquot  of the hexane layer was analyzed gas chro-
 matographically with an electron-capture detector.  As little
 as  0.5 ng of  selenium could be detected; 10 ng of selenium
 could be  determined  in a sample by extracting into 0.1 ml of
 hexane and injecting a 5-yl aliquot of the extract.  The
 method was applied to the determination of physiological
 amounts  of selenium  in human blood and urine.  River water
 samples were  also analyzed.

 In  our brief  evaluation of the gas chromatographic methods,
 known selenium samples were analyzed by the methods of Young
 and Christian13 and  Shimoishi and T6ei.l°  In a slight modifi-
 cation of their procedures, cyclohexane was used as the
 extractant for the piazselenole rather than the hexane or
 toluene used  by Young and Christian and Shimoishi and Toei,
 respectively.  From  studies of the DAN-fluorometrie method to
 be  described  later in this report, we  found cyclohexane to be
 superior  to hexane or toluene as the extracting solvent for a
 piazselenole,  primarily because of the much lower blank
 observed  with  cyclohexane.

 The minimum amount of selenium that could be detected in this
 evaluation of  the GC methods was about 0.5 ng  (per 5 yl injec-
 tion)  .  The method of Shimoishi,12 in which 4-nitro-o-
 phenylenediamine  is  used as the reagent to form the piazsele-
 nole,  was  found to be somewhat more sensitive than the method
 using  the  DAN  complex.  Because Young  and Christian13 pointed
 out that  the  DAN  complex was as sensitive to electron-capture
 detection  as  the  4,5-dichloropiazselenole of Nakashima and
Toei,9 the present results suggest that the 5-nitropiazsele-
 nole  is the most  sensitive of the piazselenoles investigated
 thus  far  to electron-capture detection.

Although  the  5-nitropiazselenole offered the most sensitivity,
 an effluent fraction that was observed in the blank produced
 a peak that overlapped the piazselenole peak and interfered
with  the measurement of peak area.  A similar observation was
made by Shimoishi12 who removed the overlapping peak by wash-
 ing the extract with hydrochloric acid solution.   The possi-
bility of back-extraction of the 5-nitropiazselenole into the
                             10

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washing solution appeared to be no problem.  It is conceiv-
able that such a washing step would have removed the overlap-
ping peak observed in our evaluation of the method.
THE DAN-FLUOROMETRIC METHOD

From the beginning of this project, we had planned to use the
DAN-fluorometric method as the primary method for the deter-
mination of selenium.  This selection was based principally
on the fact that the DAN method had been chosen as a tentative
standard method by the Intersociety Committee on Methods for
Ambient Air Sampling and Analysis.14  The essentials of this
method were introduced in 1962 by Parker and Harvey.15  The
DAN method was shown to provide more sensitivity and conve-
nience than the previously used 3,3'-diaminobenzidine method.
Since that time, the DAN method has been used for the deter-
mination of selenium in many kinds of materials.  The experi-
mental procedure is straightforward, and the sensitivity
limit has been variously reported to be 2 to 20 ng of sele-
nium.

Because the DAN-fluorometric method was the primary method of
selenium analysis used in this laboratory, the method will be
given in some detail, incorporating the modifications devel-
oped during our experimentation.

     Routine atmospheric samples were collected on 20.3-
     by 25.4-cm (8- by 10-in.) glass-fiber filters in
     high-volume samplers.  A 13- by 25-cm2 section of
     the filter (cut into 6-cm2 pieces)  was cautiously
     extracted with 30 to 50 ml of a 10:1 mixture of
     concentrated nitric and perchloric acids.  After
     boiling to reduce the volume to about 5 ml, the
     solution was diluted to approximately 50 ml with
     distilled water and the solution volume again
     reduced by boiling.  The second digestion was then
     conducted with 5 to 10 ml of concentrated hydro-
     chloric acid and the total volume reduced again.

     After appropriate treatment to dissolve the sample
     containing 0 to 1 yg of selenium, the solution was
     adjusted to pH 2 with successive additions of 10 N,
     1 N,  and 0.1 N NaOH.  The resulting solution was
     then passed through a 25-ml buret filled to the
     10-ml mark with regenerated Dowex 50 WX-8, 50- to
     100-mesh cationic exchange resin at a flow rate of
     approximately 0.5 ml/min.  The effluent was col-
     lected along with any remaining traces of selenium
     washed from the column with 25- or 30-ml of
     distilled water.  To this solution was added 0.5 ml
                             11

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      of 0.1 M EDTA,  0.5  ml  of  0.1  M NaF,  and  5 ml of
      0.1% DAN solution.   (The  DAN  was dissolved  in  u.i  i_
      HC1 to give a 0.1%  solution that was stable under
      refrigeration for about 3 days.)  The solution con
      taining the selenium was  then readjusted to PH^,
      and an additional 5 ml of 0.1% DAN was added after
      which the solution  was allowed to stand  for £  nr.

      The solution was then  transferred to a separatory
      funnel and the  piazselenole was extracted into
      exactly 10 ml of cyclohexane  by shaking  for 0.5 mm.
      After the separation of the aqueous  and  organic
      phases,  the cyclohexane layer was filtered  into a
      cuvette through a filter  paper plug  placed  in  the
      stem of the separatory funnel to remove  droplets of
      water.

      The fluorescence intensity of the sample was then
      measured at an  exciting wavelength of 380 nm and a
      fluorescence wavelength of 525 nm.   A solution con-
      taining 1.0 yg  of selenium was used  as the  refer-
      ence  standard.  When a filter fluorometer was
      employed,  a Corning No. 5970  primary (uv) filter
      was  used in conjunction with  a Corning No.  4010
      (maximum transmission  at  525  nm) secondary  filter.
      A  linear calibration curve was observed  over the
      range  of 0  to 1 yg  of  selenium in 10 ml  of  cyclo-
      hexane .

In the  procedure outlined above, the HClCK/HNOs  digestion was
used  to  leach the  selenium  from the particulate  material and
to oxidize any  selenium  present as the element to Se(IV).   It
was determined  that  the  +4  oxidation state, in solution as
selenite  (SeOs"2), was the  only form of selenium appropriate
for reaction  with  DAN to  produce the fluorescent piazselenole.
The HC1 digestion  ensured the  reduction back  to  Se(IV)  of any
selenium present as  Se(VI)  or  any  that had been  inadvertently
oxidized to Se(VI).  Experiments demonstrating the  utility  of
the hydrochloric  acid digestion are presented in Section VII
of this report.

The dilution  and  rebelling  of  the  solution after the initial
HNOa/HClOi, digestion served to  expel the  dissolved  oxides of
nitrogen and  thus  prevented nitrite ion interference.   The
EDTA and NaF  solutions were used as masking agents  to reduce
the possibility  of metal  ion interferences.  Also,  it was
found that the results were not significantly affected by
allowing the  solutions to stand in a well-lighted laboratory
for the prescribed 2-hr period.
                             12

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By using cyclohexane as the extracting solvent rather than
toluene as suggested in the Intersociety Committee method, we
were able to lower the blank markedly and to increase repro-
ducibility considerably.  After correction for the blank
value, samples containing as little as 0.01 yg of Se were
analyzed.  This detection limit represented almost an order
of magnitude reduction due to the lower blank value in cyclo-
hexane.  The detection limit offered by this modification of
the DAN-fluorometrie method was as good as, or better than,
the limits offered by the other methods of selenium analysis
evaluated in this study.

As a further evaluation of the DAN-fluorometric method, a
National Bureau of Standards Standard Reference Material was
analyzed for selenium.  This standard, "Trace Mercury in Coal"
(SRM 1630), is not certified for selenium, but a value for
selenium content of 2.1 yg/g (as Se) is included in the certi-
ficate as "information only".  Our method of analysis of this
coal sample was the usual DAN method with aqueous samples
prepared by the Schoniger oxygen combustion-flask technique.
The powdered coal samples were supported in the flask in
quartz wool contained in a platinum mesh boat.  The absorbing
solution used in the Schoniger flask was 0.5 M HN03.  Our
analysis yielded a value for selenium content of 1.6 ± 0.4
yg/g.  This value was not only lower than the reported value
of 2.1 yg/g but the precision was not as high as was desired.
Incomplete combustion of the samples could, in part, explain
the low results and lack of precision because some samples
were observed to have an insoluble tar-like residue remaining
in the flask after removal of the absorbing liquid.
                             13

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                          SECTION VI

     LABORATORY GENERATION OF SELENIUM VAPORS AND AEROSOLS


 From the outset it was  recognized that assembling and estab-
 lishing the  performance of laboratory aerosol and vapor gener-
 ators were of  prime  importance in the evaluation of candidate
 sampling devices.  Thus,  during the course of the investiga_
 tion, we constructed and  evaluated several selenium generating
 devices for  use in the  laboratory.


 NEBULIZATION AEROSOL GENERATOR

 The  first device tested was a selenium aerosol generator based
 on a DeVilbiss  No. 841  nebulizer.  In this relatively simple
 system,  the  nebulizer was used to spray a solution prepared
 by dissolving  elemental selenium in a small amount of nitric
 acid and diluting with  water or a sodium or ammonium chloride
 solution to  a  known  selenium concentration.  This nebulizer
 was  designed for inhalation therapy and is intended to produce
 particles in the size range from 1 to 10 ym when dilute aque-
 ous  solutions  are dispersed from its 500-ml solution reser-
 voir.   Filtered and  humidified air from the laboratory
 compressed air  system was passed through the nebulizer at
 2.5  1/min.   The output  of the nebulizer was diluted with clean
 compressed air  at various dilution ratios.  The relative
humidity of  the dilution  air was controlled by mixing two
dilution airstreams, one humidified and the other dried.  The
dilution chamber consisted of a 5.1-cm i.d. glass tube 51 cm
long, with three sampling ports located at intervals along
the  tube.  The  dilution chamber was vented to a hood.

Table 2  shows some preliminary results obtained for the
measured generator output at two selenium solution concentra-
tions.  The  nebulizer solution was prepared for these tests
by dissolving elemental selenium in dilute nitric acid and
diluting with water.  The aerosol was sampled for 2 hr at
2 1/min  through a glass-fiber filter followed by a bubbler
containing water.  The  glass-fiber filter was extracted with
water and the selenium was determined by the methylene blue
method.  No  selenium could be detected in the bubbler solu-
tion, indicating that most of it was collected on the filter
in particulate  form.   The results of these analyses suggested
that  reasonably satisfactory performance of the generator
system could be expected at an output concentration level of
 about 1 ng/1.  With this technique,  the concentration of
                             14

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            Table 2.   NEBULIZATION-GENERATOR OUTPUT
Concn of Se
in nebulizer
solution, mg/1
100


1

Flow rate
of dilution
air, 1/min
2.0
6.0
10.0
6.0
6.0
Dilution3
factor
0.8
2.4
4.0
2.4
2.4
Concn of Se
in generator
output, ng/1
140
110
40
0.6
0.8
    The flow rate of air through the  nebulizer  was  2.5  1/min.


 aerosol produced approached the practical  limit  that could be
 measured by the  analytical  method within a reasonable  sampling
 time.

 Although it was  possible to evaluate  field sample-collection
 devices at  the 1 ng/1  concentration  level, it  was  desirable
 to  work at  lower levels more nearly  approaching  those  that
 are expected to  occur  in the atmosphere.   We therefore con-
 ducted some preliminary experiments with nebulizer solutions
 containing  a relatively high ratio of a soluble  salt to dis-
 solved selenium.   The  principle was based  on the assumption
 that the ratio of the  salt  to selenium would be  the same in
 the aerosol particles  as in the nebulizer  solution, and that
 determination of the concentration of the  salt in  the  aerosol
 would permit a valid estimation of the concentration of sele-
 nium when the selenium was  too low to be measured  by the most
 sensitive analytical method.   To test this principle, we
 nebulized a solution containing 1 mg/1 of  selenium and 10 g/1
 of  ammonium chloride.   The  aerosol was diluted and sampled on
 a glass-fiber filter,  as in previous experiments.  The
 selenium content of the filter was determined  by the methylene
 blue  method and  the ammonium content was determined by the
 ammonia electrode  method.   The calculated weight ratio of
 ammonia (NH3) to  selenium (Se)  in the nebulizer  solution was
 3.4  x  103,  and the  ratio found by analysis of  the  generator
 output  was  4.3 x  103.   In a similar experiment in  which
 sodium  chloride was used instead of ammonium chloride  and the
 chloride  content  of the aerosol  was determined, much poorer
 agreement was obtained.

 In additional experiments we  substituted ammonium  sulfate for
 ammonium chloride  in the  nebulizer solution since  the  sulfate
 is a widely occurring  constituent of atmospheric aerosols.
The nebulizer solutions were  prepared to contain 0.01% of
 selenium  (as  Se)  and 1.0% of  NH3 added as  ammonium sulfate, a
                             15

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 ratio of NH3  to Se  of  100  to  1.  Analyses of the aerosol
 collected on  a glass-fiber filter gave ratios_of NH3  to be
 averaging about 50  to  1.   In  one set of experiments,  f^ers
 were extracted with a  measured volume of water and aliquot^
 were taken for determinations of ammonia by the Nessler method
 and for determinations of  selenium by both the DAN-
 fluorometric  and methylene blue methods.  The two methods for
 selenium gave approximately the same average value tor tne
 concentration of selenium  in  the aerosol, about 140 ng/1, but
 the precision of the measurements was poor.  Some or  tne
 variability of the  results could have been attributable_to
 instability of the  aerosol generation or to variations in
 efficiency of sampling.

 Results from  further experiments on this principle of low-
 level selenium estimation  in  laboratory-generated aerosols
 proved to be  inconsistent  and widely variable.  Also, the
 ratios of NH3  to Se found  in  the aerosol seldom agreed with
 the known ratios in the nebulizer solution.  At this  point
 this technique was  tentatively abandoned in favor of  conduct-
 ing sampling  evaluations at selenium aerosol concentration
 levels sufficient for  direct  determination by existing tech-
 niques .
DRY-SELENIUM DIOXIDE AEROSOL GENERATOR

One form in which selenium is likely to occur  in  the atmo-
sphere is selenium dioxide.  Selenium dioxide  could occur as
solid particles or as the vapor, or in an adsorbed state in
association with other particulate matter.   It is possible,
also, that selenium present in source emissions as the dioxide
might be converted in the atmosphere to selenious acid or to
another chemical form.

Thus, we set out to determine if dry selenium  dioxide could
be used as a laboratory source of selenium vapor  or particu-
late material.  As a starting point, it was  of interest to
estimate the volatility of selenium dioxide  at ambient temper-
atures.  From published data on the vapor pressure at several
temperatures,16 it was calculated that the volatility at 25°C
is about 105 ng/m3.  This value is several orders of magni-
tude higher than any reported atmospheric concentration of
selenium, and it suggested that dry selenium dioxide might be
a convenient source of selenium in the vapor form.

In a preliminary experiment to examine the practical signifi-
cance of the volatility, we placed 100 mg of selenium dioxide
between two glass-fiber filters and passed air at room temper-
ature through the filters at 2.0 1/min,  collecting samples of
the effluent air in bubblers containing water.   From the
                             16

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volatility data, as much as 2 yg of selenium dioxide was
expected in a 20-liter sample.  However, no selenium could be
detected in the bubbler solutions by the methylene blue
method.

In later experiments a more elaborate set-up was constructed
to employ dry selenium dioxide as a laboratory source of sele-
nium.  The dry-selenium dioxide generator was prepared by mix-
ing an aqueous solution of selenium dioxide and an inert
solid support, Chromosorb W, and evaporating the resulting
mixture to dryness.  The dry charge of Se02/Chromosorb W was
then placed in a suitable glass column and enclosed by glass-
wool plugs.  Provisions for temperature control were made by
wrapping the column with an insulated heating tape connected
to a variable voltage source.  For generation of selenium a
dry airstream was passed through the packed column at a fixed
rate and the effluent was sampled directly to determine its
selenium concentration.

In an effort to characterize the form in which the selenium
existed and its concentration in the effluent of the genera-
tor described above, several combinations of untreated glass-
fiber filters and midget impinger bubblers containing dis-
tilled water were used for sampling.  Typical results of the
selenium analyses for the various sampling configurations are
presented in Tables 3 and 4.  The results are given in terms
of the amount of selenium in nanograms removed by a given
sampling device per liter of generator effluent sampled.
Table 3 contains data obtained with a generator effluent tem-
perature of 74°C as measured at the glass-wool plug in the
end of the packed column.  Table 4 contains data for an
effluent temperature of 27°C  (room temperature).
  Table 3.  SELENIUM ANALYSES OF GENERATOR EFFLUENT AT 74°C
             Sampling medium
   Se  collected
   per liter of
 effluent  sampled,
	ng	
   Glass-fiber filter
     Second filter in tandem filters

   Impinger bubbler (water)
     Second bubbler in tandem bubblers

   Filter following two bubblers

   Bubbler following a single filter
                             17
       33
        1

       10
        5
        1
       0.5

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  Table 4.  SELENIUM ANALYSES OF GENERATOR EFFLUENT AT 27°C
             Sampling medium
                                          Se collected
                                          per liter of
                                        effluent sampled,
    Glass-fiber filter
      Second filter in tandem filters

    Impinger bubbler (water)

    Filter following a single bubbler
1.2
0.05

0.7

0.02
The results with two glass-fiber filters in series indicated
that the single untreated glass-fiber filter removed the sele-
nium from the generator effluent with more than 90% efficiency,
However, the impinger bubbler containing distilled water
operated with only about 30 to 60% efficiency, depending upon
the effluent temperature.  One anomalous result was the
unexpected low values for selenium retained on filters follow-
ing bubblers.  The filters were expected to retain the sele-
nium passed by the bubblers, since a low efficiency had been
found for the bubblers and a high efficiency for the filters.
It may have been that losses occurred in the sampling train
between the bubblers and the filters due to the large increase
in the humidity of the sample airstream after passage through
the bubblers.

From the results of preliminary experiments conducted with
this type of selenium generator, it was concluded that the
selenium did not exist primarily as a vapor of selenium diox-
ide or selenious acid, as expected from vapor pressure data,
but as a particulate aerosol.  Additionally, it was determined
that the total selenium concentration of the generator aerosol
was only about 2% of the selenium concentration expected in
the gas phase as calculated from equilibrium vapor pressure
data.

To further characterize the generator effluent, an Andersen
"Mini Personnel" cascade impactor was employed for the pur-
pose of approximate sizing of the particulate aerosol.  This
particular Andersen impactor provided four impaction stages
and a final filter.  To utilize the impactor, the generator
effluent was routed to a large glass jug which provided the
sampling environment and served as an enclosure for the impac-
tion device.  Several samples were taken from the generator
operating at both room temperature and 74°C under conditions
similar to those described earlier.  With the generator
                             18

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operating at room temperature, the initial sample volume of
approximately 2 m3 surprisingly yielded negligible amounts of
selenium, ^.e^, less than 20 ng.  Results from previous
experiments" Tn which the effluent was sampled with filters
and bubblers directly from the generator at room temperature
suggested that a 2-m3 sample should yield a total of selenium
in excess of 2 yg.  However, with the generator operating at
74°C, higher concentrations were obtained although these
results were lower than expected in view of earlier experi-
ments involving sampling directly from the heated generator.
Average results of the particle-size distribution for the
effluent aerosol generated at 74°C are given in Table 5 for
sample volumes of 2 to 8 m3.  The analyses for selenium on
the impactor stages were done by the DAN-fluorometrie method
with the Turner filter fluorometer.
           Table 5.  PARTICLE-SIZE DISTRIBUTION OF
             SELENIUM AEROSOL GENERATOR EFFLUENT
Impactor stage
First
Second
Third
Fourth
Final filter
Particle size,a
ym
>3.2
3.2 to 2.3
2.3 to 1.4
1.4 to 0.43
<0.43
Se,b %
18
10
9
6
57
          These size cuts were based on an assumed
         , particle density of 3 g/cm3.
          These average percentages of Se per stage
          were based on sample volumes of 2 to 8 m3
          with an average total Se concentration of
          1.7 yg/m3.
Because of the negligible amount of selenium collected with
the generator operating at room temperature and the reduced
amount collected with the generator heated to 74°C, it was
suspected that a significant amount of selenium was lost in
the tubing connections between the generator output and the
enclosure for the impactor.  However, the results of the
particle-size analysis appeared to substantiate the results
of analyses involving the sampling devices described previ-
ously.  Efficiencies of distilled-water bubblers for removing
selenium from the generator effluent were found to be 30 to
60% while untreated glass-fiber filters provided efficiencies
in excess of 90%.  In view of the significant percentage
(>50%) of aerosol particles found to be smaller than 0.5 ym,
it is reasonable to assume that most of these small particles
                             19

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 were not retained in the  midget  impinger bubblers  and,  hence,
 the analyses of the  bubbler  solutions  could be  expected to be
 low.  However,  glass-fiber  filters  appeared to  collect  tne
 small particles efficiently  and,  thus,  as  the results indi-
 cated, yielded  efficiencies  in excess  of 90%.   In  addition,
 the apparent dependence of bubbler  efficiency on generator
 temperature pointed  out in previous experiments can  be  ratio-
 nalized on the  basis of these results.  If it is assumed that
 heating the generator gave rise  to  enrichment of the smaller
 particles,  J^.e. ,  less than 0.5 to 1.0  ym,  then  bubbler  effi-
 ciencies would  be expected to decrease  with this small  parti-
 cle enrichment;  this assumption  concurred  with  experimental
 observation.

 The usual cumulative distribution plots of percent accumula-
 tion in the size  cuts versus logarithm of  the aerodynamic
 particle size were made for  the  data from  the impactor  analy-
 ses.  The curve  shapes suggested the possibility of  a bimodal
 distribution.   Although data are lacking for the large
 percentage  of particles below 0.4 ym,  the  particle distribu-
 tion appeared to  have two maxima (MMD), one below  0.4 ym and
 one above 2 ym.
DIMETHYL  SELENIDE VAPOR GENERATOR

For the evaluation of treated  filter materials  for  their  sele-
nium retention properties, a known  source of  selenium vapor
 (with no  particulate selenium) was  required.  For this purpose
the diluted vapor of a liquid  organic compound,  dimethyl
selenide, was used.  This particular organic  selenium compound
was chosen not only for its volatility  (bp =  58.2°C)  and  sta-
bility, but also for its biological significance.   The expira-
tion of dimethyl selenide into the  atmosphere can result  from
plant and animal metabolism of ingested  selenium compounds.17'11

The source of the dimethyl selenide vapor was a  100-liter
Mylar bag to which a small volume  (25 yl) of  a  dilute solution
(1.38 mg/ml) of dimethyl selenide in methanol was added during
the process of filling the bag with air.  The resulting con-
centration of selenium (as Se) in the bag was 250 ng/1.
Various candidate sampling devices were  subsequently  evaluated
by sampling the contents of the Mylar bag directly.
                             20

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                          SECTION VII

        LABORATORY EVALUATION OF METHODS OF SAMPLING
                     FOR  SELENIUM  IN AIR
 In evaluating  candidate  sampling materials for their selenium
 retention  properties,  the  approach was based on the use of
 retentive  filters  for  the  collection of selenium occurring in
 particulate  form and chemically treated filters for the col-
 lection  of selenium occurring  in gaseous or vapor form.
 Although several reagent solutions were evaluated for use in
 impinger bubblers, the conception of the most appropriate
 sampling device for field  use  was a single reagent-impregnated
 fibrous  filter for collection  of total atmospheric selenium.
EVALUATION OF TREATED AND UNTREATED FILTERS

In the initial phases of the study, we investigated the effi-
ciency of extraction of small amounts of selenium from Gelman
Type A glass-fiber  filters to which had been added known
quantities of a standard selenium dioxide solution.  On
extraction of the filters with either water or a mixture of
nitric and perchloric acids, an amount in excess of about 80%
of the added selenium was routinely recovered.  The solutions
were analyzed by the DAN-fluorometric method as outlined pre-
viously.  In one set of analyses, the nitric-perchloric acid
extracts were passed through the ion-exchange column as
suggested in the Intersociety Committee Method.  Initially,
losses of selenium  in the column were found to be as large as
25%, but further experimentation with more efficient column
washing demonstrated the losses to be usually no larger than
10%.  Also, it was  determined that the filter material itself
was not a source of interference in the selenium determina-
tions.  That is, any contribution of selenium from the glass-
fiber material was  negligible or below the detectable limit.

Subsequently, several experiments were conducted to determine
the selenium collection efficiency of glass-fiber filters,
cellulose-fiber filters, membrane filters, and glass-fiber
filters impregnated with one of three reagents.  The reagents
included lead acetate and two reducing agents—sodium sulfite
and ascorbic acid—that are known to reduce Se(IV) to elemen-
tal selenium.

These particular experiments were conducted with the selenium
aerosol generator,  described previously, that was based on
the nebulization of selenium dioxide solutions.  It was


                             21

-------
assumed that the aerosol particles formed consisted initially
of selenious acid, with more or less associated water and a
small proportion of excess nitric acid  (the selenium dioxide
in the nebulizer was in dilute nitric acid solution).  Ifc wa^
also assumed that after evaporation of  the solvent, the parti-
cles were relatively small, perhaps mostly submicron in size,
although the size distribution for this aerosol generator was
not determined.

Thus, glass-fiber filters  (Gelman Type  A) and Millipore fil-
ters (0.8-ym pore size) were compared for their retention of
aerosol particles from the selenium aerosol generator.  The
nebulizer solution containing 0.01% selenium  (as Se) was
prepared by dissolving elemental selenium in dilute nitric
acid.  First, it was shown that known amounts of standard
selenious acid solutions added to either type of filter were
essentially completely recovered (80%, or more) by extraction
with either water or a nitric-perchloric acid mixture.  The
output of the aerosol generator was sampled on 2.54-cm diam-
eter filters at 2 1/min for 10 min.  The filters were
extracted with the acid mixture and analyzed by the DAN-
fluorometric method.  The glass-fiber filters appeared to be
about twice as effective as the Millipore filters in retaining
the aerosol particles.  The significance of this result was
uncertain since the particle-size distribution of the
nebulizer aerosol was not established; however, it did suggest
that a sizable proportion of the aerosol particles were sub-
micron in size, as had been previously assumed.

Cellulose-fiber filters (Whatman No. 42) were also compared
with glass-fiber filters for sampling the output of the gener-
ator.  Concentration values (0.03 yg/1)  calculated from deter-
mination of selenium collected on the cellulose-fiber filters
were only about 25% of the concentration values found with
glass-fiber filters (0.12 yg/1).   Both types of filters were
extracted with dilute nitric acid.   The reason for this
unexpected large difference was not clear.   However, the
results of these determinations,  unlike most of the results
that follow for the impregnated filters, were consistent and
reproducible.

The experiments with impregnated filters were generally less
than satisfactory because of the  lack of reproducibility of
the results.  The poor reproducibility seemed to be most
likely caused by difficulties  in  controlling the output of
the nebulizer.   There were, however,  indications that the
reducing agents, sodium sulfite and ascorbic acid,  may have
brought about some reduction of selenious acid to elemental
selenium.   The amounts of selenium  found on  these filters bv
extraction with water and determination  by  the DAN-
fluorometric method were consistently lower  than the amounts
                             22

-------
 found  on  unimpregnated  glass-fiber  filters,  indicating  that
 some of the  selenium was  in  a water-insoluble  form.  However,
 the results  calculated  as concentration  of  selenium  in  the
 generator output were not reproducible.

 There  was evidence,  also,  that  lead acetate  in  the filters
 may have  reacted with the selenious acid (or selenium dioxide)
 in the generator output to form a water-insoluble product.
 Concentration  values calculated from analyses of the filters
 were in some instances  lower for lead acetate-impregnated
 filters extracted  with  water than for filters extracted with
 dilute nitric  acid.   This result suggested  that an acid-
 soluble but  water-insoluble  product may  have been formed.
 Also,  a small  amount of selenious acid added directly to a
 lead acetate-impregnated  filter was not  recovered by subse-
 quent  extraction with water.

 Treated filters were evaluated  also by sampling the effluent
 of the dry-SeOa generator that  was  described earlier in this
 report.   Glass-fiber filters were treated with  special solu-
 tions  of  silver nitrate in one  case and  lead acetate in
 another.   The  treatment solutions were essentially those
 described by Natusch et a_l. 19 for treatment  of  filter mate-
 rials  for the  collection  of  hydrogen sulfide.   All treated
 filters were used  as back-up filters in  tandem  arrangements
 with untreated glass-fiber filters.   The results of the pre-
 liminary  experiments were  somewhat  encouraging; these results
 are shown in Table 6.   The data indicated that  the first
 untreated filter consistently removed the selenium present in
 particulate  form,  and that the  untreated and lead acetate-
 treated back-up filters essentially collected no additional
 selenium.  However,  the AgNO3-treated back-up filter appeared
 to collect an  amount of selenium in excess of the amount in
 particulate  form retained  on the first filter.  This tentative
 result for the silver nitrate-treated filter suggested that
 such a treated filter might be  useful for retention of sele-
 nium present in the  ambient  atmosphere in the vapor form.
 Also,  these  data suggested that the dry-selenium dioxide
 generator was  producing an effluent consisting  of selenium
 dioxide in both the  vapor  and particulate forms, contrary to
 other  indications  that  the selenium dioxide  was essentially
 all particulate (see  p. 17).

 However,  after additional  experiments  of a more diagnostic
nature were  conducted,  the favorable  results first obtained
with the  silver nitrate-treated filters were found to be sub-
 ject to much uncertainty.  The  fluorescence  readings obtained
 for blank  silver nitrate-treated filters proved to be highly
variable.   Indeed, results obtained with silver nitrate-
treated filters using the  dry-selenium dioxide  generator as
the source of  selenium  yielded  efficiencies  from less than
                             23

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      Table 6.   SELENIUM RETENTION  OF  TREATED FILTERS  FOR
           SAMPLING THE DRY-Se02  GENERATOR  EFFLUENT
                     (PRELIMINARY RESULTS)
Glass-fiber filter treatment
First untreated filter
Second untreated filter
First untreated filter
Second Pb(OAc)a filter
First untreated filter
Second AgNO3 filter
Se retention
(ng/1 of generator effluent)
1.25
<0.1
1.25
<0.1
1.25
1.45



 10%  to  about  70%  for  selenium retention relative  to  the  first
 untreated  filter  in a tandem arrangement.  After  several such
 analyses,  it  was  concluded that the silver nitrate-treated
 filters possessed little, if any, reproducible selenium-
 retention  capability.  A similar result was obtained for
 silver  nitrate-treated filters used in sampling a dilute
 vapor of the  organic  selenium compound, dimethyl  selenide.
 The  250-ng/l  vapor source of selenium used in these  evalua-
 tions was  the Mylar dilution bag described earlier.   The
 resulting  retention efficiencies of the treated and  untreated
 filters for dimethyl  selenide vapor were essentially the same
 and  typically less than 5%.

 In view of the latter  results, it appeared that the  initial
 conclusions concerning the nature of the effluent from the
 dry-selenium  dioxide  generator were appropriate.  That is, no
 definitive evidence was found to suggest that the selenium
 present in the generator effluent was in any form other  than
 particulate.
EVALUATION OF ACTIVATED CARBON

Sorbent tubes containing activated carbon were also used in
an effort to collect dimethyl selenide vapor from the Mylar
dilution bag.   (The activated carbon was obtained from Mine
Safety Appliances Company and was the product approved by the
National Institute for Occupational Safety and Health for
sampling organic vapors.)  However, DAN analyses of samples
obtained in this manner indicated selenium retention effi-
ciencies typically less than 5%.  In these experiments, dis-
tilled water was used to desorb the dimethyl selenide from
the carbon.  The low result did not necessarily mean that the
                             24

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 dimethyl  selenide  vapor was  collected  on  the  carbon with  low
 efficiency,  but it may have  been  that  the overall  recovery
 process of desorbing and analyzing  the selenide had a  low
 efficiency.   This  conclusion was  supported in part by  the
 work  of Lewis et al.2° on the collection  of volatile selenium
 compounds expired  from plants.  These  authors demonstrated by
 using Se75 in radiotracer experiments  that activated carbon
 was^effective in retaining volatile selenium  compounds.   In
 addition  they found distilled water and 50% aqueous ethanol
 to be greater than 90% effective  in desorbing the  volatile
 selenium  compound  from the carbon.   In light  of the fact  that
 our sampling efficiencies were very low with  essentially  the
 same  sampling technique,  it  is probable that  after desorption
 the selenide was not efficiently  oxidized to  the +4 oxidation
 state that is necessary for  reaction with the DAN  in the
 analytical procedure.   The results  were essentially the same,
 very  low,  even when the samples were digested in the mixture
 of oxidizing acids,  nitric and perchloric.  However, uncer-
 tainties  in  the digestion treatment included  possible  losses
 of dimethyl  selenide  by volatilization.
EVALUATION  OF  BUBBLERS

Because  experiments  using  treated  filters, untreated filters,
and  activated  carbon for sampling  the vapor of dimethyl sele-
nide were essentially unsuccessful,  subsequent experiments
employed bubblers  containing water,  dilute nitric acid, or
lead acetate solution for  trapping the vapor.  These experi-
ments were  essentially  no  more  successful than earlier experi-
ments; however,  a  few meaningful conclusions were drawn from
these experiments  with  bubblers.

As the data in Table 7  demonstrate,  the bubbler solutions
that were digested with an oxidizing acid mixture consistently
had higher  values  for selenium  collection efficiency compared
to the undigested  samples, even though all efficiencies were
low.  The percent  efficiencies were  based on a 5-min sample
at a 1 1/min flow  rate  from a selenium source of  250 ng/1 or
a total  theoretical  sample of 1.25 yg of selenium.  The solu-
tions from  the bubblers were analyzed by the usual DAN method
and, although the  differences in the percent efficiencies of
the digested and undigested samples  appeared small, the dif-
ferences were experimentally significant.

These data  suggested  that, although  the results with the dif-
ferent bubbler solutions were similar, a bubbler could possi-
bly be used for sampling dimethyl  selenide with subsequent
digestion with an  oxidizing acid and analysis by the DAN
method.   The low yields in these experiments were also
                             25

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          Table  7.  COLLECTION EFFICIENCIES OF BUBBLER
                  SOLUTIONS FOR  (CH3)2Se VAPOR
Bubbler solution
H20
1 N HN03
2% Pb(OAc)2
H20
1 N HNO3
2% Pb(OAc) 2
Digestion step
HNOs/HClOi,
HNOa/HClO^
HNOs/HClO^
None
None
None
Efficiency, %
8
8
8
2
3
1
  probably due to losses of dimethyl selenide through its vola-
  tility and the inefficient oxidation of dimethyl selenide to
  the +4 oxidation state necessary for DAN analysis.

  These results also paralleled those of Lewis et aJ.2° in
  their radiotracer evaluations of bubbler solutions for sam-
  pling volatile selenium compounds.   They were not able to
  retain significant amounts of selenium in a variety of liquid
  media:  aqueous solutions of EDTA,  KOH, NH^OH, lead acetate,
  and KMn04-KOH;  sodium ethylate in ethanol;  the organic solvent
 benzene;  and diaminobenzidine (DAB)  in hydrochloric acid solu-'
  tion.

 The choice  of the  appropriate oxidizing conditions to effect
 the necessary oxidation of selenide  to selenite on a filter
 or in  a bubbler is  not a straightforward matter.   However
 such an oxidation would be necessary for the analysis of a
 volatile  selenide by  the DAN  method.   Dimethyl selenide is
 oxidized  to  dimethyl  selenoxide  by  the common strong oxidiz-
 ing agents and  to dimethyl selenide  dihalide by active halogen
 oxidizing agents.   Neither of these  compounds is  likely to
 react  in  significant  yields with DAN in the fluorometric
 method  of analysis.   The  yield of the  reaction of these com-
 pounds , or dimethyl selenide  itself, with the oxidizing diges-
 tion acids to give  Se(IV)  would  also be expected  to be low!
 Thus, the efficient reaction  of  dimethyl selenide on a
 reagent-impregnated filter surface to  give  Se(IV)  for  DAN
 analysis  does not appear  feasible.  However,  the  trapping  of
 a volatile selenium compound  on  a solid sorbent with oxidation
 and analysis at a later time  appears to be  an  avenue  for  fur-
 ther investigation.

 In conjunction with the evaluation of bubbler  solutions for
 the retention of dimethyl  selenide vapor, bubbler solutions
were also tested for  retention of selenium in particulate
 form.  Using the effluent of  the  dry-selenium dioxide
                             26

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generator, evaluations were conducted on aqueous bubbler solu-
tions containing 10% lead acetate  (suggested by the work of
Pillay et al.2:) , 2% AgNO3, 0 .1 M HNO3, or distilled water.
The results of the selenium analyses conducted on these
bubbler solutions after exposure to the generator effluent
are presented  in Table 8.  The most significant result
appeared to be the high efficiency of the dilute nitric acid
bubbler relative to the others.  Also, it was significant
that the lead  acetate bubbler performed poorly.  These results
suggested that a dilute acid solution would be most appropri-
ate for use in a back-up bubbler for the sampling of ambient
atmosphere for selenium.
     Table 8.  RETENTION OF SELENIUM FROM THE DRY-Se02
           GENERATOR BY VARIOUS BUBBLER SOLUTIONS
    Bubbler solution
 Concn of  Se  in generator effluent,
  as  indicated by analysis of the
	bubbler solutions, ng/1	
      Distilled H20
      0.1 M HNO 3
      2% AgNO3
      10% Pb(OAc)2
                3.8
                5.6
                1.2
              <0.2
                             27

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                        SECTION VIII

         SELENIUM SAMPLING OF THE AMBIENT ATMOSPHERE
           AND INVESTIGATION OF THE CHEMICAL FORMS
                   OF ATMOSPHERIC SELENIUM


 In a preliminary evaluation of the Intersociety Committee DAN
 procedure  for analysis of atmospheric particulate material,
 25-hr samples were collected on glass-fiber filters in a high-
 volume sampler and were briefly digested with nitric and
 perchloric acids and analyzed.  Values for selenium concentra-
 tions of 4.6, 5.1, 3.9, and 6.8 ng/m3 were obtained at various
 times.  These results were similar to those reported in the
 literature for selenium concentrations found in other parts
 of the country.  Pillay, for example, found concentrations of
 total selenium in the range of 3 to 9 ng/m3.21  Others have
 found similar small concentrations.22"26
WATER-SOLUBLE AND WATER-INSOLUBLE SELENIUM IN PARTICULATE
MATERIAL

The analysis of atmospheric particulate samples was explored
further on other occasions.  Samples from the ambient atmo-
sphere outside the laboratory were collected on 20.3- by
25.4-cm (8- by 10-in.) glass-fiber filters with a high-volume
sampler.  One sample was cut into several squares of known
area.  Individual squares were extracted with either water or
a mixture of nitric and perchloric acids.  Prior to analysis
by the DAN-fluorometric method, some of the extracts were
passed through an ion-exchange column to remove impurities
and others were not.  Water extracts analyzed directly without
the ion-exchange treatment showed strong effects of interfer-
ences and did not give the normal fluorescence spectrum of
the DAN piazselenole.  Water extracts treated by ion-exchange
gave more nearly normal spectra, showing the effects of remov-
ing interfering substances.  A blank portion of the filter
extracted with water gave a normal low relative fluorescence
intensity, indicating that the interfering substances were
present in the atmospheric particulate material and not in
the filter material.  The atmospheric concentration of sele-
nium estimated from these results for this particular sample
was 0.36 ng/m3.

Acid extracts analyzed directly without the ion-exchange
treatment gave more nearly normal spectra than the water
extracts and an estimated selenium concentration value of
                             28

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0.93 ng/m3.  With the ion-exchange treatment, a concentration
value of 0.65 ng/m3 was obtained.  Previous results with stan-
dard solutions of selenious acid indicated that as much as
25% of the selenium could be lost in the ion-exchange treat-
ment if extreme care were not exercised.

The results of these experiments indicated that a portion of
the selenium in the atmospheric particulate material was solu-
ble in water and that acid treatment was required for the
dissolution of a substantial additional portion.  Thus, the
selenium apparently occurred in at least two different states
of chemical combination.  These forms of particulate selenium
were most probably selenium dioxide  (or selenious acid depend-
ing on the amount of moisture present) and elemental selenium.
The selenium dioxide  (or selenious acid) would have been
readily soluble in water yielding Se(IV) in solution, but
elemental selenium would have required acid dissolution to
give Se(IV) in solution for reaction with DAN.  If it is
assumed that the element and the dioxide were the only chemi-
cal forms of particulate selenium, as the data indicate, then
it can be tentatively concluded from the analyses that each
contributed approximately equally to the total selenium
associated with the particulate material.
ATMOSPHERIC SAMPLING WITH FILTERS AND BUBBLERS

In additional experiments, untreated glass-fiber filters and
distilled-water bubblers of the gas-dispersion type were
employed for collecting ambient air samples.  The samples
were taken over a 6-day period within a laboratory open to a
continuous flow of ambient air.  During this period the aver-
age total particulate concentration was determined to be
approximately 60 yg/m3.  The sample volumes ranged from 43 to
51 m3.  All samples were treated with a moderate HNOa/HClCH
(10:1) digestion step prior to routine analysis.  Results of
the DAN-fluorometric analyses for the selenium content of
these atmospheric samples are presented in Table 9.  These
results, in general, paralleled those obtained with the dry-
SeOa generator; however, the high value for selenium retained
on glass-fiber filters following the distilled-water bubblers
was unexpected.  It could be that the conditions of high
humidity following the bubblers contributed to the increased
efficiency of the glass-fiber filters for removal of atmo-
spheric selenium.  Consequently, this particular result was
in contrast to results of similar experiments conducted with
the dry-selenium dioxide generator.  In those experiments
filters following bubblers retained relatively small amounts
of selenium even though the bubbler efficiency was low.
                             29

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                 Table 9.   SELENIUM CONTENT OF
                  AMBIENT  ATMOSPHERIC SAMPLES
Sampling medium
Glass-fiber filter
Second filter in tandem filters
Distilled water bubbler
Filter following a bubbler
Second filter in tandem filters
Se found,
ng/m3
1.2
0.2a
0.7
2.3
0.7
         This  value  is  uncertain because of  its  proxim-
         ity to  the  blank.


 SELENIUM ANALYSIS  OF SIZED ATMOSPHERIC PARTICULATE MATERIAL

 From the data in Table 9,  the  total  atmospheric selenium con-
 centration during  this particular  sampling  period  was  esti-
 mated to be  3 ng/m3.   The  selenium content  of  the  particulate
 matter was estimated by analysis to  be 20 yg/g.  These  results
 agree favorably with similar analyses reported  in  the  litera-
 ture.  In particular,  a recent publication  by Natusch  et al.27
 suggested that  several toxic elements, including selenlumT"
 were preferentially concentrated into the smallest airborne
 particles of  fly ash derived from  coal-fired power plants.
 Natusch's selenium  analyses by atomic absorption spectrometry
 ranged from 11  to 59  yg/g  for  airborne fly-ash  particle sizes
 from 11.3 to  1.1 ym,  respectively.

 A  similar experiment was conducted in this  laboratory  to
 determine whether or not such  a preferential concentration  of
 selenium in small particles exists for ambient  atmospheric
 particulate matter.  To  facilitate this study we employed an
 Andersen  cascade impactor which provided nine size cuts
 (eight stages and a  final  filter)  over a particle-size  range
 of 0.4  to 10  ym.  The  particulate  matter on each stage  was
 weighed  and analyzed for selenium  in the usual manner.
 Table  10  shows  the results of  the  measurements.  During the
 sampling  period  the average total  particulate concentration
 was  approximately 41 yg/m3 for a sample volume of  700 m3
 The  particle-size distribution was found to be log-normal
 with a mass median diameter (MMD)  of approximately 1 ym.
 Although  the results of the selenium analyses appeared  some-
what uncertain because of the  low  value for the  0.95-to-1.8 ym
 size cut, one can conclude that this experiment was not  posi-
 tive evidence for the phenomenon of preferential concentration
of selenium into the smaller particles (<0.5 ym) of the


                             30

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        Table  10.  ANDERSEN CASCADE  IMPACTOR SAMPLING
                  OF THE AMBIENT ATMOSPHERE
_ Stage
0
1
2
3
4
5
6
7
Filter
Size,a ym
>10
6.4 to 10
4.3 to 6.4
2.9 to 4.3
1.8 to 2.9
0.95 to 1.8
0.57 to 0.95
0.38 to 0.57
<0.38
Particulate
weight per stage
(% of total)
10.5
7.2
6.5
7.2
9.2
11.2
11.8
19.7
16.5
Se content
per stage
(% of total)
<5
<5
<5
<5
35
<5b
40
7
<5
       flow rate was  25  1/min at  25°C for these size cuts.
   This result  is uncertain.
ambient  atmospheric  aerosol.   In  contrast to the preferential
concentration phenomenon  as cited by Natusch,27 the present
results  tentatively  suggest that  the selenium in a multisource
atmospheric  aerosol  may be most abundant in particle sizes
close to the MMD.
BACKGROUND LEVEL OF ATMOSPHERIC SELENIUM

Other analyses of high-volume atmospheric samples revealed a
significant difference between the selenium content of the
particulate material in a heavy industry area and that in a
less polluted area.  An air  sample taken in an area of moder-
ate particulate concentration  (on the Institute premises) was
found to have a selenium content corresponding to 2.0 ng/m3.
The average total particulate concentration of this sample
was 44 yg/m3.  From these figures the selenium content of the
particulate matter was calculated to be 47 yg/g.  However,
analysis of a sample taken in an area of heavy industry  (North
Birmingham) gave a result of 2.6 ng/m3 for selenium concentra-
tion when the average particulate concentration was 350  yg/m3.
These figures suggested an average selenium content of
7.4 yg/g in the ambient particulate matter in this area.
Thus, it appears that the industry in this area of high  par-
ticulate pollution is not a  source of atmospheric selenium.
It is probable that the selenium content of the ambient  atmo-
sphere in this area is simply the background amount that is
present in the urban Birmingham atmosphere.
                             31

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DETERMINATIONS OF Se(VI) WITH DAN

In further efforts to characterize the chemical forms in
which selenium occurs in the ambient atmosphere, a question
arose concerning the applicability of the DAN-fluorescence
method of selenium analysis to the determination of selenium
in the +6 oxidation state.  Because the necessary oxidation
state for DAN analysis is + 4, it was considered important to
determine if the method described by the Intersociety Commit-
tee for the selenium analysis of atmospheric particulate
matter would account for any selenium present as selenate.
Thus, experiments were conducted in our laboratories with
known Se(VI) solutions  (weighed as Na2Se04) treated with the
usual HN03/HC104  (10:1) digestion step, and subsequently ana-
lyzed by the DAN method.  The results of these experiments
are presented in Table 11.  These results demonstrated that
known Se(VI) solutions analyzed in the conventional manner
gave essentially no indication of any selenium content; how-
ever, a known solution of Se(IV) analyzed simultaneously gave
the expected result.  Therefore, the results of a selenium
analysis of a particulate sample containing selenate would
have been expected to be low by the DAN method as it existed
at that time.  Consequently, additional experiments were
directed toward finding a suitable reducing agent to incorpo-
rate in the DAN procedure.  The proper reducing agent would
ensure that all of the soluble selenium present as selenate
would be reduced to selenite (Se(IV)) for reaction with DAN.
A desirable reducing agent would also be compatible with the
HNOs/HClOit digestion mixture used to oxidize any elemental
selenium that might be present in the particulate sample to
Se(IV).  The utility for this purpose of a hydrochloric acid
digestion subsequent to the HN03/HC10it digestion was suggested
by an article on the Se(IV)/Se(VI) electrochemical couple.28
It was indicated in this study that the oxidation of Se(IV)
to Se(VI) could be effected in aqueous solution with chlorine
and that Se(VI) could in turn be reduced to Se(IV) by treat-
ment with concentrated hydrochloric acid.  Thus, a hydro-
chloric acid digestion step was incorporated in the analysis
of known Se(VI) solutions by the DAN method.  These results
are also contained in Table 11.   It was concluded that the
hydrochloric acid digestion was effective in quantitatively
reducing Se(VI) to Se(IV) for the DAN analysis.  The final
group of results in Table 11 indicate that the simultaneous
determination of Se(IV) and Se(VI) was successful when the
additional hydrochloric acid digestion was used but not when
only the conventional HNOa/HClO.* digestion was used.  In the
latter case, only the selenium added as Se(IV) was determined
and not the Se(VI).  As a result of these findings, we
included the hydrochloric acid digestion step in subsequent
analyses of atmospheric particulate samples.
                             32

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       Table  11.  EFFECT OF DIGESTION TREATMENT ON THE
        DETERMINATION OF SELENIUM BY THE DAN METHOD
__ Sample
Se(IV) (control)
Se(VI)
Se(VI)
Se(VI)
Se(IV) (control)
Se(VI)
Se(VI)
Se(VI)
Se(IV) (control)
Se(IV) + Se(VI)
Se(IV) + Se(VI)
Amount of Se
taken, yg
1.00
1.00
0.75
0.50
1.00
1.00
0.75
0.50
1.00
0.5 + 0.5
0.5 + 0.5
Amount of Se
found, yg
0.97
<0.02
<0.02
<0.02
0.94
0.95
0.73
0.52
0.96
0.97
0.50
Digestion
treatment
HNOa/HClO.*
HN03/HC10.+
HNOs/HClO.*
HNOs/HClOit
HC1
HC1
HC1
HC1
HaO only
HNOa/HClOit/HCl
HN03/HCKK only
ANALYSIS OF ATMOSPHERIC  PARTICULATE MATERIAL FOR Se(VI)

These  findings  also  suggested  a  means  of estimating the rela-
tive amount of  selenium  present  in a particulate sample as
Se(VI).  Consequently, we  conducted selenium analyses of atmo-
spheric particulate  samples with and without the hydrochloric
acid digestion  step  in an  effort to determine whether Se(VI)
was present in  the samples.  Although  a limited number of
determinations  were  made for this comparison, the results
with the hydrochloric acid digestion step were not signifi-
cantly different  from those obtained without it; that is, the
results of both methods  agreed within  the experimental repro-
ducibility  (2.0 ± 0.5 ng/m3).  On the  basis of these limited
findings, therefore, we  may conclude that Se(VI)  (as selenate)
is not a primary  form of atmospheric selenium.
ACCIDENTAL OXIDATION OF  SELENIUM  IN PARTICULATE MATERIAL TO
Se(VI)

Particulate samples on glass-fiber filters  that were  treated
with an extended HClOij/HNOa digestion  step  and no hydrochloric
acid digestion step appeared  to contain essentially no  sele-
nium by conventional DAN analysis.  However,  an identical
sample treated in the same manner but  with  the incorporation
of the hydrochloric acid digestion step yielded the expected
amount of selenium  (^1.5 ng/m3).  The  extended HClOu/HNOs
digestion step was characterized by the complete bleaching
and dissolution of the initially black atmospheric particulate
                              33

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matter by repeated heating to dense fumes of perchloric acid
and to near dryness, whereas the usual HClO^/HNOa digestion
did not always result in complete dissolution of the particu-
late material because the samples were not taken to dense
fumes of perchloric acid.  It appeared that such rigorous
oxidizing conditions may have oxidized all the selenium pres-
ent in oxidation states lower than +6 to Se(VI).  Subsequently,
the sample treated with the hydrochloric acid digestion step
gave the expected result because all the selenium that was
previously oxidized to Se(VI) was reduced by the hydrochloric
acid to Se(IV),  the oxidation state appropriate for DAN
analysis.

These apparently low results obtained in the case of an
extended digestion with nitric and perchloric acids have been
interpreted by other workers29'30 to be due to losses of sele-
nium by volatilization.  The present results, however, suggest
that the selenium was not lost at all but that it was simply
oxidized to a form inappropriate for DAN analysis.   Thus,  the
inclusion  of a hydrochloric acid digestion step not only
allows DAN determination of Se(VI),  but it also minimizes the
probability of low results due to over-oxidation in the
HClCU/HNOs digestion step.
                             34

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                          SECTION  IX

             CONCLUSIONS  CONCERNING THE NATURE OF
                     ATMOSPHERIC SELENIUM


 The  statement is  made  in the  description of  the "Tentative
 Methods  of Analysis  for  Selenium  Content of  the Atmospheric
 Particulate Matter"  that "selenium may exist in the atmosphere
 most^probably as  the element,  the oxide, or  as the sele-
 nite".11*   it is  also stated that  hydrogen  selenide is likely
 to have  a  short  lifetime because, like hydrogen sulfide, it
 is rapidly oxidized  in air.

 A similar  conclusion can be drawn from the experimental evi-
 dence  presented  in this  report.   The  present results indicate
 that the selenium contained in particulate matter consists of
 approximately equal  amounts of water-soluble and water-
 insoluble  forms  of selenium.   These forms  are most likely
 elemental  selenium as  the water-insoluble  form and selenium
 dioxide  (or selenite)  as the  water-soluble form.  Results of
 experiments designed to  indicate  the  presence of Se(VI)
 failed to  establish  selenate  as a component  of atmospheric
 aerosols.

 Results  of experiments with the dry-selenium dioxide generator
 suggested  that little  or no selenium  dioxide would exist as a
 true vapor in an  ambient atmosphere.  Indeed, it appears
 likely that atmospheric  selenium  dioxide exists as, or is
 associated with,  particulate  matter.  These  results also indi-
 cated  that conventional  particulate sampling would be suffi-
 cient  for  atmospheric  selenium dioxide sampling with negligi-
 ble  losses due to volatilization.  Experimental results also
 demonstrated the  greater efficiency of glass-fiber filters
 relative to distilled-water bubblers  for sampling the effluent
 of the dry-selenium  dioxide generator.  These observations
 and  conclusions contradict statements by West as quoted in a
 recent article by Pupp e_t al. 3 l   West reported that gross
 sampling errors would  occur for selenium dioxide if only
 particulate  sampling was employed.  Also, West suggested that
bubblers filled with water are highly efficient samplers of
 selenium dioxide.

 Pupp et al.3 1  also reported a  few measurements on the equilib-
bium vapor  concentration of selenium  dioxide.  The method
employed for the  measurement was  a weight-loss Knudsen system,
which  assumes  a weight loss occurs because of the effusion of
a vapor through an orifice.   The  values reported by Pupp
                              35

-------
agree, in general,  with the concentrations found in the
effluent of the dry-selenium dioxide generator employed in
this study.  However,  in the present work, it was found that
the selenium dioxide existed almost entirely in an aerosol
form and not in the expected vapor form.

Because the burning of fossil fuels, coal in particular, has
been cited as the largest source of atmospheric selenium,4
the chemical form of selenium emitted in this process is
likely to be the predominant selenium species present in an
ambient atmospheric aerosol.  Recent studies have indicated
that the selenium present in the effluent of a coal-burning
power plant is exclusively in the elemental state.32  It was
suggested in these studies that the elemental selenium
resulted from the reduction of selenium dioxide (or selenious
acid) by sulfur dioxide.  Also, it was determined that greater
than 90% of the emitted selenium existed in the vapor phase.
This result was observed in the case of a coal-burning steam
plant with electrostatic precipitators that gave a fly-ash
removal efficiency in excess of 99.5%.

However, it is possible, perhaps likely, that the elemental
selenium vapor released from power plant stacks is converted
rapidly to particulate form—by condensation or adsorption,
or by some other process.  This view was also presented by
Weiss et al.3 3 who studied selenium-to-sulfur ratios in dated
Greenland ice cores and found that sulfur has increased con-
siderably during recent times, whereas, selenium has remained
fairly constant.  Such results suggest that atmospheric sele-
nium is particulate in nature and that input of elemental
selenium vapor into the environment should be a local effect.

The manufacture of glass has also been pointed out as a sig-
nificant source of atmospheric selenium.k  At one location
where large quantities of colored structural plate glass are
produced, the stack effluent was analyzed.  The stack efflu-
ent analysis showed that a large part of the metallic sele-
nium used in the batch composition was volatilized during
melting, and consequently appeared in the effluent as red,
amorphous selenium metal.  The particle size was reported to
be less than 0.35 ym.1*

The possibility of organic selenium compounds being present
in the ambient atmosphere must at least be considered because
of the metabolic fate of selenium ingested by both plants and
animals.  The volatile organic compound, dimethyl selenide,
is an example of a selenium compound known to be a metabolic
product expired by plants and animals.17'18  Although this
compound is one of the most probable forms of airborne organic
selenium, the contribution of dimethyl selenide or any organic
selenium compound to total atmospheric selenium in an urban
                             36

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 nvironment is likely to be very small.  Indeed, no evidence
  r the presence of an organic selenium vapor—or any signif-
1?a^t selenium vapor, for that matter—was obtained in this
study.  However, it should be pointed out that concentrations
o  elemental selenium vapor could have been underestimated in
tnis investigation.  Although the data of Pillay et al.21'34
suggested that approximately 50% of atmospheric selenium is
in a vapor form, bubblers used in the present study for atmo-
spheric sampling indicated in one case a maximum of 20% sele-
nium present in the vapor form.

Thus, it appears that the selenium present in the ambient
atmosphere exists predominantly as, or in association with,
particulate matter, and that atmospheric selenium is most
likely in the chemical form of the element and the dioxide
(or selenite).
                             37

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                             39

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22.   Hashiomoto, Y.,  and J. W. Winchester.  Selenium in the
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      April 1967.

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                       41

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                                   TECHNICAL REPORT DATA
                            (Please read Instructions on the reverse before completing)
   DEPORT NO.
   EPA 650/2-74-124
                                                           3. RECIPIENT'S ACCESSIOWNO.
 4. TITLE AND SUBTITLE
                                                           5. REPORT
   Development of a Sampling Method for Total
   Atmospheric Selenium
                                                                               CODE
         . Barrett
Herbert C. Miller
                                                           8. PERFORMING ORGANIZATION REPORT NO.
                                                             SORI-EAS-74-353
                                                             Project 3128 - XIII
 I. PERFORMING ORGANIZATION NAME AN.O ADDRESS
  Engineering and Applied Sciences Department
  Southern Research Institute
  2000 Ninth Ave. South
  Birmingham, Ala.  35205
                                                        10. PROGRAM ELEMENT NO.

                                                          1AA010
                                                        11. CONTRACT/GRANT NO.

                                                          68-02-1220
 12. SPONSORING AGENCY NAME AND ADDRESS
   EPA,  NERC
   Research Triangle Park,
                                                           13. TYPE OF REPORT AND PERIOD COVERED
                                                            Final. June 1973/May  1974
                            C.  27711
                                                        14. SPONSORING AGENCY CODE
 15. SUPPLEMENTARY NOTES
 16. ABSTRACT
        The primary purposes of this contract were  to  develop a collection technique
   for the quantitative collection of total atmospheric  selenium and to characterize
   the forms in which selenium occurs in the atmosphere.
        Retentive filters were used to collect  particulate material containing selenium;
   chemically treated filters for the collection  of gaseous or vaporous selenium were
   evaluated.  Laboratory generators were setup for the  evaluation of filter materials.
   The 2,3-diaminonaphthalene (DAN) fluorometric  method  was established as the primary
   analytical technique to determine selenium.  Air particulate samples were analyzed
   for selenium and procedures were studied for determining relative amounts of
   selenium present in different oxidation states.   Selenium aerosol and vapor
   generators were set up, characterized and used to evaluate other sampling devices.
        Selenium appears to be present in ambient air  predominantly as or with
   particulate matter; thus it was tentatively  established that conventional particulate
   sampling suffices to collect the bulk of atmospheric  selenium.  Selenium vapors
   appear to make negligible contributions to total  atmospheric selenium; however,
   concentrations of elemental selenium could have  been  underestimated.
        The most probable chemical forms of atmospheric  selenium were determined to  be
   the element and the dioxide (or selenite).
                               KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
                                             b.IDENTIFIERS/OPEN ENDED TERMS
                                                                        c. COSATI Field/Group
  Air Particulate Sampling
  Selenium Sampling and Analysis
  Selenium - Vapor and Aerosol
  Ambient Air Selenium
                                           Fluorometry  with
                                           2,3-diaminonaphthalene

                                           Methylene  Blue Method

                                           Gas Chromatography
                                             19. SECURITY CLASS (ThisReport)
                                              Unclassified
                                                                     21. NO. OF PAGES

                                                                         47
                                              !0. SECURITY CLASS (This page)
                                              Unclassified
                                                                     22. PRICE
EPA Form 2220-1 (9-73)
                                           42

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