influence of oxalic acid on the catalytic s

138
al., 2000; Yao et al., 2004). Kawamura and
Kaplan (1987) found that the distribution <strong>of</strong>
dicarboxylic <strong>acid</strong>s measured in gasoline and
diesel motor exhausts is similar to that <strong>of</strong> urban
air samples. The concentrations <strong>of</strong> these <strong>acid</strong>s in
motor exhausts are 28 (gasoline) and 144
(diesel) times higher than the average <strong>of</strong>
atmospheric concentrations.
Because <strong>of</strong> their low vapour pressures
and high solubility in water, dicarboxylic <strong>acid</strong>s
are predominantly present in atmospheric
aerosols, cloud droplets, fog droplets and
precipitation (Chebbi and Carlier, 1996;
Kawamura and Ikushima, 1993; Souza et al.,
1999). Oxalic <strong>acid</strong> is the single most abundant
organic compound identified in ambient
aerosols. It is found both in PM10 and PM2.5
fractions. Its concentrations in PM10 vary from
0.38 to 1.84 µg/m 3 and in PM2.5 from 0.01 to
1.53 µg/m 3 (Wang et al., 2002, 2007; Huang et
al., 2005; Yao et al. 2004; Yang and Yu, 2008).
Concentrations <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> in the range 0.1 –
40 µmol/dm 3 have been measured in the
tropospheric aqueous phase samples from
different areas in the world (Chebbi and Carlier,
1996; Kawamura et al., 1996b, Löflund et al.,
2002).
Very little is known so far about the
<strong>influence</strong> <strong>of</strong> carboxylic <strong>acid</strong>s on the catalytic
S(IV) oxidation by O2. Current laboratory
studies <strong>of</strong> chemical reactions involving S(IV)
species and organic ligands have focused on the
iron-catalysed autoxidation (Grgić et al., 1995;
Grgić and Poznič, 1998; Grgić et al., 1998;
Grgić et al., 1999; Wolf et al., 2000). It has been
found that organic ligands such as oxalate,
acetate and formate inhibit this process. The
<strong>influence</strong> <strong>of</strong> some low molecular weight
monocarboxylic <strong>acid</strong>s (formic, acetic, glycolic,
lactic) and dicarboxylic <strong>acid</strong>s (<strong>oxalic</strong>, malic,
malonic) on the Mn(II)-catalysed S(IV)
oxidation has also been investigated (Grgić et al.
2002, Podkrajšek et al. 2006, Wilkosz and
Mainka, 2008). It has been established that
monocarboxylic <strong>acid</strong>s inhibit the oxidation <strong>of</strong>
S(IV). From among dicarboxylic <strong>acid</strong>s, <strong>oxalic</strong>
<strong>acid</strong> slows down the S(IV) oxidation, while
malic and malonic <strong>acid</strong>s have practically no
<strong>influence</strong>.
The investigated process is a very
complex free radical chain reaction. Its
mechanism and kinetics are so sensitive to the
reaction conditions that even a slight change in
them can cause a change <strong>of</strong> the dominant path
<strong>of</strong> the reaction course, and thus lead to diverse
results. Thus, despite the studies cited above the
effect <strong>of</strong> carboxylic <strong>acid</strong>s on the metal catalysed
oxidation <strong>of</strong> S(IV) in atmospheric water is still
poorly understood and more work in this area is
needed. The purpose <strong>of</strong> the present work was to
study the <strong>influence</strong> <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> on the Mn(II)catalysed
S(IV) oxidation under the conditions
representative for <strong>acid</strong>ified atmospheric water in
heavily polluted areas.
Materials and Methods
All reagents used were <strong>of</strong> analytical grade
(Merck). Milli-Q water was used for preparation
<strong>of</strong> all solutions. Stock solutions <strong>of</strong> Mn(II) and
<strong>oxalic</strong> <strong>acid</strong> were prepared from MnSO4⋅H2O and
H2C2O4⋅2H2O, respectively. The S(IV) solutions
were prepared freshly before each run by
dissolving Na2SO3 in water which was
deoxygenated by bubbling high purity argon
through the Milli-Q water for at least 30 min.
The initial pH <strong>of</strong> the solutions was adjusted with
H2SO4. The source <strong>of</strong> oxygen for oxidation <strong>of</strong>
S(IV) was synthetic air.
Kinetic experiments were conducted in
a 500 cm 3 glass cylindrical reactor with four
inlet connectors for: pH electrode, introducing
reagents, thermometer and teflon tube for
sample sipping. The reactor was filled with 450
cm 3 <strong>of</strong> the S(IV) solution <strong>acid</strong>ified to the
required pH and containing appropriate amount
<strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>. The reactor was protected from
light and immersed into a thermostat to
guarantee a constant temperature <strong>of</strong> 25 ± 1 o C.
The air was introduced at the bottom <strong>of</strong> reactor
through a ceramal at a rate <strong>of</strong> 100 ± 2 dm 3 /h.
Under such conditions the gas and liquid phases
were good mixed and the reaction took place in
the kinetic regime, i.e. the global rate <strong>of</strong> S(IV)
oxidation was limited by the rate <strong>of</strong> chemical
reaction, not by the diffusion.
To start the reaction, the air flow was
turned on and just after that the Mn(II) solution
was injected into the reactor. At selected time
intervals the concentration <strong>of</strong> S(IV) was
measured by UV-VIS (Shimadzu, Model UV-
2101 PC) spectrophotometer equipped with
Sipper 260 (Model L) - using flow cell method.
The sipping time was set to 5 s, and the slit
width was set to 2.0 nm. The S(IV)
measurements were carried out at wavelength
203 nm for the initial pH 3.5 and 205 nm for the
initial pH 4.0 and 5.0. The pH measurements
were performed by an Orion pH meter (Model
710A) combined with a glass electrode. The
concentration <strong>of</strong> Mn(II) was determined by
AVANTA PM atomic absorption spectrometer
<strong>of</strong> the GBC.
Conditions <strong>of</strong> experiments were as
follows: [S(IV)] ≈ 1·10 -3 mol/dm 3 , 1·10 -6 ≤
[Mn(II)] ≤ 1·10 -5 mol/dm 3 , 1·10 -6 ≤ [H2C2O4] ≤
1·10 -4 mol/dm 3 , 3.5 ≤ the initial pH ≤ 5.0, T =
298 K.