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

-------
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:


-------
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

-------