V-/EPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-085 June 1981
Project Summary
Sulfur Dioxide Oxidation
Reactions in Aqueous
Solutions
L. R. Martin, D. E. Damschen, and H. S. Judeikis
The kinetics and mechanisms of
more than one hundred reactions
involving the aqueous phase oxidation
of sulfur dioxide and nitrogen oxides in
mixed catalyst/oxidant systems at pH
= 0 to 3 were investigated.
The systems investigated involve
different redox reaction types: SO2/
O2; SO2/NO,; SO2/H2O2; SO2/O3;
NOx/O3; NO»/H2O2. The S02/02 sys-
tem demonstrated little reactivity un-
less a catalyst was present. The most
effective metal catalysts were FE+++
and Mn++ with a complex synergism
between Fe and Mn.
In the SOz/NO, system the nitrite
ion caused a rapid reaction producing
N2O. The reaction did show slight
inhibition by toluene and hexene, but
was unaffected by metal ion catalysts.
The SOz/H2O2 reaction in water is
extremely fast and insensitive to catal-
ysis or inhibition.
The SOz/Qa reaction is also rapid
and similarly insensitive to catalysis or
inhibition.
Nitrite oxidation by both ozone and
hydrogen peroxide is rapid. The pH
dependence of these two reactions
was determined.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Our understanding of the atmospheric
chemistry of sulfur and nitrogen oxides
is important for establishing control
strategies both for the primary pollutants,
sulfur dioxide and nitric oxide, and for
their aerosol formation products, sul-
fates and nitrates. Research in the past
ten years has identified the importance
of several gas phase reactions leading
to sulfate or nitrate formation. It is only
recently that liquid droplet phase mech-
anisms have been addressed as a pos-
sible explanation for the wintertime
presence of sulfate and nitrate in regions
where photochemical oxidant potential
is obviously low.
This project has addressed, in a sys-
tematic fashion, over one hundred
aqueous phase reactions of potential
significance in sulfate and nitrate for-
mation. Environmentally important
metal catalysts, commonly found oxi-
dants (02, O3, H202) in the ambient air,
and potential inhibitors or accelerators
of the main reactions, S02 — SOI and
NOX -~ NOs were studied for their re-
action rates.
Procedure
The apparatus used consisted of an
ultraviolet light source, a Durrum D-103
stopped flow system, monochromator
(PAR 1208) and two detection systems
interfaced to a minicomputer. The detec-
tion systems used, depending on reac-
tions conditions, were (1) a PAR 1205D
Silicon Intensified Target Detector linked
to a PAR 1205A Optical Multichannel
Analyzer and (2) an RCA IP28 phototube
maintained at -650v by a Fluke 405
Power Supply. The tube is linked to a
Biomation 802 Transient Recorder to
record waveshape during selected peri-
ods of time. The minicomputer used was
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a Cromemco Z2 which utilizes BASIC
language.
Fresh aqueous solutions were pre-
pared daily using 18 megohm resistivity
water obtained from a Milli-Q water
purification system. .HCI was used to
maintain a pH of less than 4. The pH was
measured before each run using an
Orion 701 A digital ionalyzer. The water
for the S02 solutions was deoxygenated
by passing a steady flow of nitrogen
through it. For the ozone reactions an
Orec Model 03V5-0 ozonator was used
to prepare 1x10~4M solutions. The re-
ductant is placed on one side of the
stopped flow system while the oxidant
plus catalysts or inhibitors is placed on
the other. The capability to detect
1x10"5M of the absorbing species ex-
isted; usually solutions were prepared
at concentrations of 1 x1 0"3M to 1 x1 0MM
to obtain a better signal to noise ratio.
Results
The following rate expressions have
been determined during the course of
this project:
(1 ) For the S02 + 02 reaction at pH = 2
d(Svl)/dt = 1 970 (Mn++) + 82(Fe+++)
[1+ 2.67x1 Q
The reaction is zeroth order in 02.
(2) For the SO2 + N02 reaction:
d(Svl)/dt = 142(H+)1/2 (HNOz+NOi)
(S02-Aq+HSOi)
(3) For the S02 + H2O2 reaction:
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
AGE"CY
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