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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
l
METAL SPECIATION IN FLUE GASES
AUD
WORK PLACE ATMOSPBERES
o,
Klockow
R.D. Kaiser, K. Larjava, J. Reith, V. Siemens
Institute of Spectrochemistry and ~~~lied
Spectroscopy, (ISAS)
Dortmund, Federal German Republic
The determination of metals in technical and ambient aerosols is
a field of analytical chemistry related, e.g., to emission
control, investigation of atmospheric processes, or to effects
research. In this context not only the total content of a
certain metallic element in a volume of air or in a dust sample,
respectively, is of interest, but also its distribution between
different phases or over different size ranges and its occurrence
in definite chemical forms (species).
In aquatic chemistry the determination of dissolved and
particulate metal species has been given considerable attention
to for many years. In emission measurements or air quality
studies, however, attempts towards metal speciation are scarce
and have concentrated mainly on gaseous metal compounds.
Flue gases and work place atmospheres as well as ambient air are
complex mixtures of severa1 minor and major gaseous and
particulate compounds, socalled aerosols, in which droplets
and/or solid particles are suspended in a carrier gas.
Consequently, metal compounds also may be present in gas, liquid
or solid phase in such aerosols, their partitioning being a
function of the aerosol generating process involved.
Taking ambient air as an example, sea spray, soil dust raised by
winds, volcanic eruptions, or cement manufacturing are typical
emission sources with primary particle production. In these cases
material, through mechanical action, is emitted directly in
particulate state (predominantly bigger or equal to 1 micron
diameter).
During, e-g., fossil fue1 combustion or ferrous and nonferrous
metallurgical operations secondary particles (smaller than 1
micron
diameter) are formed besides primary
ones by
condensation f
rom vapour af ter its escape f
rom high temperature
sources. In technkal as well as in ambient aerosols large
fractions of typical heavy metals (Pb, Cd, Zn) may be found in
this submicron size range, often enriched at surfaces of small
primary particles.
Homogeneous or heterogeneous chemical transformations of airborne
trace gases (gas-to-particle conversion) likewise lead to
secondary particulate matter and are of particular relevance for
a number of atmospheric compunds such as S02, N02, NH3 and also
metal alkyl species.
A speciation scheme for characterization of a metal containing
aerosol may be divided into a physical and a chemical part. The
physical part includes investigation of the partitioning of the
metal under study between the different phases or determination
of the metal in different particle (droplet) size ranges.
The
chemical part deals with identification and quantification of
well defined chemical forms of the respective metal in gas,
liquid and solid phase.
Metal speciation in aerosols first of al1 requires proper
sampling techniques which allow reliable separation and
preconcentration of gaseous and particulate metal compounds
without artifact formation. In principie such techniques are
based on the interaction of inertial and viscous forces
(impactors, filters), electrical forces. (electrical mobility
analyzers), and diffusion
(diffusion batteries, denuders,
filters).
Especially, diffusion processing has found wide
application in aerosol sampling and will be shown to be a
promising approach to metal speciation in flue gases also.
In particulate matter collected from workplace air on a suitable
substrate metal, speciation may be achieved by thermal or X-ray
techniques.
Among these, X-ray diffraction is the most
straightforward and unambiguous method, because it yields not
only compound but also phase (lattice) specific information.
Electron spectroscopy is well suited for particle surface
analysis, whereas laser microprobe mass analysis
(LAMMA)
has been
successfully applied to single particle characterization
including depth profiling. A simple microchemical technique
employing phase selective leaching procedures can be used for
determination of Ni, Co or Cr species in microgram amounts of
collected particulate matter. This will be shown using dust from
metal plating workplaces and from a nickel refinery as examples.
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