United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-88/018 Apr. 1988
Project Summary
Oxidative Degradation of
Organic Acids Conjugated with
Sulfite Oxidation in Flue Gas
Desulfurization
Y. Joseph Lee and Gary T. Rochelle
Organic acid degradation conjugated
with sulfite oxidation has been studied
under flue gas desulfurization (FGD)
conditions. The oxidative degradation
constant, k,2, is defined as the ratio of
organic acid degradation rate and
sulfite oxidation rate times the ratio of
the concentrations of dissolved S(IV)
and organic acid. It is not significantly
affected by pH or dissolved oxygen in
the absence of Mn or Fe. However. k,2
is increased by certain transition metals
such as Fe, Co, and Ni and is decreased
by Mn and halides. Lower dissolved
S(IV) magnifies these effects. A free
radical mechanism was proposed to
describe the kinetics. Hydroxy and
sulfonated carboxylic acids degrade
approximately three times slower than
saturated dicarboxylic acids; while
maleic acid, an unsaturated dicarbox-
ylic acid, degraded an order of mag-
nitude faster. A wide spectrum of
degradation products of adipic acid
were found including carbon dioxide
(the major product), smaller dicarbox-
ylic acids, monocarboxylic acids, other
carbonyl compounds, and hydrocar-
bons.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research L aboratory. Research
Triangle 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
Currently, limestone scrubbing is the
dominant commercial technology for flue
gas desulfurization (FGD). The perform-
ance of limestone scrubbing is chemi-
cally limited by two pH extremes: (a) low
pH near the gas/liquid interface, which
decreases the solubility and absorption
rate of SO^ and (b) high pH near the
liquid/solid interface, which decreases
the solubility and dissolution rate of
limestone. Organic acids that buffer
between pH 3.0 and 5.5 enhance S02
removal efficiency and limestone utiliza-
tion at concentrations of 5 to 10 mM.
Adipic acid was the first organic acid
buffer additive successfully and gener-
ally applied to FGD processes up to
commercial scale. It has been replaced
commercially by waste dibasic acid
(DBA), a waste from adipic acid and
cyclohexanone production, containing
primarily adipic, glutaric, and succinic
acids. DBA was found to be as effective
as adipic acid. Other potential alterna-
tives include hydrocarboxylic acids and
sulfonated carboxylic acids. They are of
interest because of reduced volatility and
potentially lower degradation rates.
In addition to the expected loss of
organic acid additive by entrainment of
solution in waste solids, loss by chemical
degradation and coprecipitation is also
observed. Chemical degradation, which
is conjugated with sulfate oxidation, is
the most important mechanism of buffer
loss under forced oxidation conditions.
Assuming that both oxidation and
degradation are free radical reactions
proceeded by a common radical, previous
investigators proposed a simple kinetic
model which can be expressed as:
d[A]/dt = k,2tfA]/[SdVMd(dIS(IV)l/dt) (1)
where [A] and [S(IV)J stand for organic
acid and sulfite/bisulfite molar concen-
trations, respectively, and superscripts
"d" and "t" denote "dissolved" and
-------
"total," respectively. The most probable
free radical here is SOi because of its
reactivity toward alcohols which are
inhibitors of sulfite oxidation. It has also
been cited as the active species in the
decarboxylation/oxidation of organic
acids by peroxydisulfate (S20s2~).
Valeric acid, glutaric acid, and hydro-
carbons from ethane to butane have been
identified as degradation products of
adipic acid. Low pH, especially in the
presence of Mn, reduced the degradation
rate of adipic acid. It was also observed
that high dissolved S(IV) reduced the
degradation rate.
It is desirable to understand the
degradation kinetics, the mechanism,
and the additional environmental impact
caused by degradation. This work covers
the degradation kinetics, mechanism,
and products. Since decarboxylation is
the major degradation pathway and since
it is decarboxylation that significantly
affects the buffering capacity, C02
evolution rate instead of the actual
degradation rate is of greater interest.
The oxidative degradation constant, ki2,
will be used for most kinetic studies,
since this constant is simply the degra-
dation rate normalized with the oxidation
rate and the concentrations of organic
acid and dissolved S(IV).
Experimental Apparatus and
Procedure
Since organic acid degradation is
conjugated with sulfite oxidation, and
experimental design is required that
permits measuring the rates of organic
acid degradation and sulfite oxidation
simultaneously. Two approaches are
used for this study: one oxidizes calcium
sulfite slurry; and the other, sodium
sulfite solution. The slurry system has
a large capacity of sulfite to allow
significant degradation of organic acid
with relatively low [S(IV)]d, typical of
actual scrubber conditions. The solution
system provides better control of dis-
solved [S(IV)]. Two reactors have been
used to study organic acid degradation
during sulfite oxidation: one is an open
semibatch reactor, while the other is a
closed semibatch reactor which enables
monitoring the gas-phase degradation
products, primarily carbon dioxide. Fig-
ure 1 diagrams the experimental appa-
ratus with the closed system reactor.
In a typical slurry experiment, 1.0 M
synthesized CaSO3 solids were slurried
and oxidized in a solution containing 10
mM organic acid and 0.1 M CaSO4 2H20
pH PH
Controller Meter
Ice Bath
Condenser
Sampling Port
Recorder
Temperature
Controller
Water Bath
/V2/0
Figure 1. Closed system apparatus.
seed crystals. All experiments were
performed at a constant temperature of
55°C. The pH was maintained constant
throughout a given experiment. In a
typical sodium sulfite solution experi-
ment, ~1 mM dissolved S(IV) was main-
tained constant by titrating with sodium
sulfite solution during the oxidation of
S(IV) to S(VI). 0.3 M sodium sulfate was
added to prevent dramatic change in the
ionic strength. The oxidation rate of S(IV)
was reflected by the titration rate of
sodium sulfite solution.
Total S(IV) was analyzed by iodimetric
titration of the slurry sample. Filtered
samples were reheated to 55°C, the
experimental temperature, and the pH
was adjusted back to the original pH
(4.50-5.50 ±0.01) with sodium sulfite or
air oxidation. Then iodimetric titration
was used to measure the dissolved S(IV).
Carbon dioxide was monitored continu-
ously by an infrared C02 detector.
Hydrocarbons, organic acids, and liquid-
phase degradation products were ana-
lyzed by ion chromatograph exclusion
(ICE, Dionex 14), gas chromatograph (GC,
Varian 3700), and/or gas chromato-
graph/mass spectrometer (GC/MS,
Finnigan 4023).
Kinetics
Most of the experiments were per-
formed with adipic or glutaric acid. Adipic
acid is effective, nonvolatile, and has
been extensively tested up to commercial
scale. Glutaric acid is the major compo-
nent of waste DBA from adipic acid
production. The kinetic results discussed
below are applied to adipic and glutaric
acids only, unless specified.
Conjugation with Sulfite
Oxidation
No degradation of organic acid is
detected when sulfite oxidation is
inhibited by adding inhibitors such as
sodium thiosulfate and hydroxyl com-
pounds and by depleting oxygen or S(IV)
species. When the oxidation rate con-
trolled by the sodium sulfite feedrate
increases from 0.0082 to 0.024 M/hr,
the oxidative degradation constant, ki2,
only changes the 1.17x10"3 to 0.99x10"3.
In slurry experiments, the sulfite oxida-
tion rate is around 1 M/hr, about 100
times greater than the aforementioned
rates. An oxidative degradation constant,
kiz, of 2x10"3 could be obtained with the
introduction of the estimated dissolved
S(IV). The higher value of kizfor the slurry
system is probably due to the addition
of Fe to the system through the disso-
lution of calcium sulfite and calcium
sulfate solids. Therefore, the organic acid
degradation rate is directly proportional
to the sulfite oxidation rate.
-------
pH and Dissolved S(/V)
A number of slurry experiments sug-
gest that the degradation rate increases
with increasing pH. However, dissolved
S(IV) is significantly affected by pH in the
slurry system (Table 1). Experiments with
the sodium sulfite solution system
without Mn or Fe at pH 5.0 suggest that
the apparent effect of pH on ki2 in the
slurry system is due to the variation of
dissolved S(IV) as pH changes (Figure 2).
In addition to affecting dissolved S(IV) in
the slurry system, pH also affects the
distribution of sulfite and bisulfite and
the degree of protonation or dissociation
of organic acid, which might be important
in the degradation kinetics. Further
studies showed no effect of pH on Ki2
when the dissolved S(IV) was kept ~1
mM for three different catalyst environ-
ments (Figure 3). Figure 3 also shows
that, in the presence of 0.1 mM Fe, kia
decreases as the pH increases over 5.0.
Transition Metals
Figure 3 shows that kia is increased
by Fe and decreased by Mn. Sodium
sulfite solution experiments show that it
is the ratio [Fe]/[S(IV)]d (Figure 4) or
[Mn]/[S(IV)]d (Figure 5) that controls ki2.
In Figure 4, a background level of 0.0007
mM Fe is assumed according to the cor-
relation result which will be discussed
later. The decrease of k12 with increasing
pH in the presence of 0.1 mM Fe (Figure
3) may be due to decreasing solubility
of Fe at higher pH.
The first series of transition metals
(except Sc and Zn) were studied in
sodium sulfite solution. Among these,
Co, Ni, and probably Cu appear to
increase the oxidative degradation con-
stant, ki2, while Ti, V, and Cr are inert
(Table 2). TI showed a minor inhibiting
effect on the degradation of organic acid
in the slurry system. The effect of TI was
not detectable in the sodium sulfite
solution system, probably because of
interference with the measurement of
dissolved S(IV).
Halides
It has been reported that the decom-
position of carboxylate salts during
sulfite oxidation is suppressed by the
presence of 0.5 to 3 M chloride in the
aqueous absorbent. Table 3 lists the
resulting ki2 values of adipic acid with
different concentrations of chloride,
bromide, and iodide. Manganese effects
are included for the convenience of
comparison. In sodium sulfite solution,
Table 1. Steady State Solution Composition in CaSO3 Slurry as a Function of pH, Closed
Reactor. 55°C. No Organic Acid
pH
4.0
4.0
4.5
45
5.0
5.0
50
5.0
Fe'
mM
0.15
0.13
0.12
0.12
0.02
0.02
-------
c
a
<«
a
c
.5
1
I
0.5
02
_ I
\
A 0.1 mM Fe
O No Catalyst
/ mM Mn
I _
o
4.5
5.0
pH
5.5
6.0
Figure 3.
Effect of pH on the degradation of glutaric acid: sodium sulfite solution, closed
reactor. 55°C, dissolved S(IV) ~1 mM.
0.0040
_ 0.0035
0.0030
0.0025
S 0.0020 '
o
0.0015
000 0.02 0.04 0.06
[Fe]/lS(IVf\a
0.08
0.10
0.12
Figure 4. Effect of Fe on the degradation of adipic acid (sodium sulfite solution), closed
reactor. 55°C.
reacting with thiosulfate than with
organic acid. This effect is not very strong
in the presence of high dissolved S(IV)
because of the significant reaction of
S(IV) with free radicals.
Oxygen
Figure 6 shows that there is no effect
of oxygen partial pressure on ki2, with
or without addition of Mn or Fe. There-
fore, the reaction order of oxygen in
degradation and oxidation reactions
should be equal and cannot be detected
in the oxidative degradation experiments.
Alternatives
Table 4 summarizes the oxidative de-
gradation constants of several potential
organic acid buffer additives. Maleic acid
degrades about seven times faster than
saturated dicarboxylic acids, probably
due to the presence of conjugated double
bonds. In reducing the 'degradation of
maleic acid, Mn is not as effective as for
saturated dicarboxylic acids. Fe has little
effect on maleic acid degradation. On the
other hand, hydroxycarboxylic acids
degrade about three times slower than
dicarboxylic acids. The smaller oxidative
degradation constants of hydroxyacids
probably are not due to the formation of
an intramolecular hydrogen bond
because of he indistinguishable degra-
dation behavior between 3- and 2-
hydroxybenzoic acids. Sulfosuccinic acid
behaves more or less like hydroxycarbox-
ylic acids except that Fe has no signif-
icant effect on the degradation of sul-
fosuccinic acid.
Degradation Products
The degradation products of adipic acid
are widely distributed. They can be
classified as dicarboxylic acids, monocar-
boxylic acids, hydroxycarboxylic acids,
keto-acids, furans, hydrocarbons, and
carbon dioxide (Table 5). The degradation
product distribution is also a strong
function of catalyst environment. Carbon
dioxide is the primary degradation prod-
uct for all cases. In the presence of 1
mM Mn, glutaric and valeric acids are
major products. In the absence of Mn,
glutaric semialdehyde is the major liquid-
phase product. Although oxygen would
be required to generate a smaller dicar-
boxylic acid, the dissolved oxygen in the
presence of 1 mM Mn should be prac-
tically zero because of the mass transfer
controlled sulfite oxidation. Therefore,
Mn may be an effective carrier of oxygen
to the degradation product(s).
Mechanism
Figure 7 gives a mechanism proposed
to rationalize the experimental observa-
tions. This mechanism is based on two
assumptions: (1) only sulfate radical (Eq.
2) and manganic ion (Eq. 3) react with
organic acids to cause significant deg-
radation, primarily decarboxylation; and
(2) other reactions of organic radicals,
except the degradation of organic acids,
are negligible.
kp2'
Kd1
2ki3 \ kp2 + kp2'
kp2 + kp2'
1 +
kp3
[Fe]
-------
('*
kd2 ki4'
kd1 ki2'
[Mn]\
000/4 .
ki4' [Mn] ki4 [Fe] \
1 +
ki3 [S(IV)J ki3 (S(\V)] (16)
Recognizing that sulfate radical (Eq. 2)
and manganic ion (Eq. 3) react with
organic acids at much slower rates than
with S(IV) (Eq. 8, 7) and assuming that
intermediates reach steady state concen-
trations, Eq. (1) and the mechanism
proposed above lead to an expression of
the oxidative degradation constant, ki2,
(Eq. 16) with three variables: dissolved
S(IV), Mn, and Fe. Sixty-six data points
collected from sodium sulfite solution
experiments were used for corrlation.
These data can be grouped as (1) no Mn/
no Fe, (2) no Mn/FeX), (3) MnX>/no
Fe, and (4) MnXVFeX). A background
concentration of iron (Fe0) is introduced
because Fe or a comparable catalyst
must be present to initiate the reaction.
Therefore, Eq. (16) ends up with seven
adjustable parameters determined by
nonlinear regression:
kd1/(2ki3) = 0.0041
kp2Y(kp2 + kp2') = 0.227
kp3/kil = 0.0184
(kd2/kd1 )(ki4Vki2') = 0.85
ki4'/ki3 = 11.93
ki4/ki3 = 0.65
[Fe0] = 0.0007 mM
The experimental and calculated values
of kia are compared in Figure 8.
Acid Reactions
Peroxydisulfates have been exten-
sively studied for oxidizing organic
compounds, especially organic acids. The
sulfate radical anion is a well established
intermediate in the thermal and photo-
lytical decomposition of peroxydisulfates
and is believed to be responsible for the
degradation of organic compounds (Eq.
2). Oxidative degradation of organic acids
by Mn3+ has also been studied (Eq. 3).
Additional evidence demonstrates that
sulfate radical reacts with organic acid
under FGD conditions:
1. Sulfate radical can be easily
derived from peroxymonosulfate,
S0s~2, an expected intermediate,
by analogy to peroxydisulfate,
s2o82-.
2
g
<3
§
$?
*jj
° °004
0.0002
0.0000
00
20
25
f
Flgure 5
Table 2.
Effect of Mn on the degradation of adipic acid (sodium sulfite solution)- closed
reactor, 55°C, pH 5.
Effects of Transition Metals on *12 in Sodium Sulfite Solution by COZ Evolution
from 10 mM Adipic Acid at pH 5.0, 55°C
CQ
^dative Degradation Constant,
,
10
o
0.03
0 1
03
1.0
3.0
100
0.81
0.84
0.76
0.82
0.99
1 60
3.30
089
1.18
2.48
0.87
0.89
093
1.00
1.31
1 33
0.86
0.90
086
0.84
1 10
7.23
1.43
1.46
1.54
0.99
1 03
1.02
1 02
2,3
2,3
1,2
3,4
2,3.4,5
State
2,3,6
Table 3.
Cone.,
Effects of Ha/ides on £12 in Sodium Sulfite Solution by C02 Evolution from 10
mM Adipic Acid at pH 5.0, 55°C
Oxidative Degradation Rate Constant,
103
MnSOt
0
0.1
0.3
1
10
30
100
300
2.0
. 0.8
0.6
0.3
2.7 3.1
23 01
20
1.5
02
1 0
07
03
0.3
Wlth 1'° mM Fe
2. The ratio of sulfate radical reacting
with organic acid to that reacting
with S(IV) is consistent between
experimental results and literature
predictions.
3. Previous work on oxidative degra-
dation of organic acids by sulfate
radical indicates that decarboxyla-
tion is the major pathway, which
is consistent with experimental
observations (Table 5).
The hydroxyl radical is probably not
responsible for organic acid degradation
at FGD conditions. It is known to react
-------
with organic acids and can be generated
from sulfate radical by reacting with
water or hydroxyl ion. However, reaction
of hydroxyl radical with organic acids
does not result in simple decarboxylation,
and at pH less than 8.5 the reactions
making hydroxyl radicals are negligible.
Sulfur Compounds Reaction
Sulfite radical. The reaction of sulfite
radical with O2 (Eq. 11) provides a way
to "activate" oxygen for the oxidation of
S(IV). The sulfite radical is not very
reactive toward organic compounds but
reacts at a much higher rate with oxygen
and at an even greater rate with itself.
The latter reaction is probably the major
termination reaction when the reaction
rate is controlled by mass transfer of
oxygen and has also been proposed as
a termination reaction. The stable termi-
nation products were reported to be
sulfur trioxide and dithionate which will
be hydrolyzed to sulfate and sulfite.
Sulfate radical. The sulfate radical is
a very reactive on-electron transfer
oxidizing agent. Some correlated and
literature reaction rate constants are
compared in Table 6. The reaction of
sulfate radical with HSO3" is believed to
be at least 2.5 times faster with S032~.
Therefore, the sulfate radical will react
with S(IV) faster at lower pH. In contrast,
the sulfate radical reacts with organic
acid slower at lower pH. Therefore, a
smaller oxidative degradation constant,
kia, is expected at lower pH, although
experimental data do not confirm this
projection.
Peroxymonosulfate. The peroxymon-
osulfate radical (SO5~) is logically derived
from the reaction of sulfite radical with
oxygen (Eq. 11). The S05" thus formed
reacts with S(IV) to produce sulfate
radical, sulfite radical, and peroxymono-
sulfate (HSCV) in parallel reactions (Eq.
12, 13). The coexistence of these two
reactions suggests a minimum k12 cor-
responding to the fraction of S0s~ that
reacts with S(IV) to form sulfate radical.
It is suggested (Eq. 16) that the direct
production of SO<~ from S05~ (Eq. 13) is
not affected by Fe, while the indirect
production of S(V from SOi (Eq. 5)
through the formation of S052" interme-
diate (Eq. 12) is catalyzed by Fe. There-
fore, SOi is the precursor of S04", which
is the major species responsible for the
degradation of organic acid. Since the
formation of S05" is inevitable, the
degradation of organic acid in conjuga-
tion with S(IV) oxidation is also inevitable.
5
I
O
0.5
0.2
I I I I I
with 0.1 mM Fe
I I I _
No Catalyst
with 1 mM Mn
till
I 1 I
0.2
0.4
0.6
0.8 0.9
Po2(atm)
Figure 6. Effect of oxygen on the degradation of glutaric acid: sodium sulfite solution, closed
reactor, 55°C. dissolved S(IV) ~1 mM.
Table 4. Comparison of Organic Acids Buffer Additives in Terms of k,2 at pH 5.0, S5°C,
in Sodium Sulfite Solution by COZ Evolution
Organic Acid
Adi pic
Glutaric
4-Hydroxybutyric
Hydroxyacetic
Malic
3 -Hydroxybenzoic
2-Hydroxybenzoic
Benzole
Sulfosuccinic
Maleic
Oxidative Degradation Rate Constant, *12
[S(IVJ\a 1 mM No
mM Mn Catalyst
1.0
1.0
1.0
1.0
6.0
2.0
3.0
1.5
1.5
5.0
0.3
0.3
0.1
0.2
03
0.4
0.8
0.1
3.6
1.0
1.0
0.3
6.7
xlO3
0.1 mM
Fe
3.0
3.0
1.4
1.0
1.4
0.3
7.0
Iron Reactions
In the proposed mechanism, free
radicals are primarily generated by the
reaction between ferrous ion and perox-
ymonosulfatefEq. 5). It is well known that
ferrous ion reacts with peroxides to form
ferric and a free radical formed by
splitting the peroxide bond. Also, the free
radical is thought to be sulfate radical
instead of hydroxyl radical.
Ferrous ion also reacts with sulfate
radical (Eq. 9) at a significant rate.
Therefore, Fe generates sulfate radical
by reaction with peroxymonosulfate (Eq.
5) but also depletes sulfate radical by
direct reaction (Eq. 8). As a result, Fe can
inhibit or enhance organic acid degra-
dation depending on solution conditions.
Manganese Reactions
By analogy to the reaction between
ferrous ion and sulfate radical (Eq. 9),
manganous reacts with sulfate radical
(Eq. 10). The latter reaction is necessary
for the mechanism to match the obser-
vation that manganese decreases ki2.
The reaction rate constant is estimated
to be 1.8x1010. To provide a route for
manganous regeneration, manganic ion
should also react with S(IV) (Eq. 7) by
analogy to the reaction between ferric
ion and S(IV) (Eq. 6). The ratio of the
-------
Table 5. Degradation Products from 80% Degradation of 10 mM Adipic Acid in Calcium
Sulfite Slurry with 0.1 mM Mn at pH 5.0. 55°C in Terms of the Percentage of
the Initial Concentration (mM C)
Dicarboxylic
C6 Adipic
120)
C5 Glutaric
(1.5)
C4 Succinic
(0.1)
Monocarboxylic
Valeric
(0.7)
Butyric
(0.4)
Hydroxycarboxylic
5-Hydroxyvaleric
(2.5)
4 -Hydroxybutync
(2.5}
Hydrocarbons Others
Tetrahydro
2,5 furan
Dicarboxylic"
Glutaric
Semialdehyde
(6.7)
4-Oxo-
pentanoic
10.3)
Butane Succinic
(1.0) Semialdehyde'
Furane
(06)
C3 Malonic
(2.3)
C2
C1
Formic
(2.0)
Propane*
Ethane*
Methane" Formaldehyde"
CO 2(49)
"less than 0.1 %
kdO
Degradation
stable products
12)
13)
(4)
(5)
16)
17)
(8)
(9)
(10)
(11)
(12)
113)
(14)
(15)
Figure 7. A mechanism for the oxidative
degradation of organic acid
conjugated with the oxidation of
S(IV) to S(VI).
reaction rates of Mn3* with organic acid
and Mn3* with S(IV), kd2/ki2', was found
to be 0.0006, which is an order of
magnitude smaller than the sulfate
radical counterpart, kd1/ki3 (0.0082).
Halide Reactions
By analogy to Mn, halides inhibit
organic acid degradation by reacting with
sulfate radical. The experimental and
calculated values of ki2 with bromide
present are compared in Figure 8.
The correlated value of ki4(Br)/ki3
(1.26) leads to a prediction of 1.7x109 as
the reaction rate constant between SO/
and Br~. This constant has been
measured as (3.5±0.4)x109.
Not enough iodide data are available
for the prediction of the reaction rate
constant between iodide and sulfate
radical. However, halides tend to react
with organic compounds in decreasing
order (CI2>Br2>!2), while halides tend to
react with SO/ in increasing order
(CI~
-------
70.0
£
-2
3
a / o
a
o
I
<§
0.1
Mn=0. Fe=0
O Mn=0, Fe=0.
Mn>0,
O Mn>0, Fe>0
A Br
Table 6. Rates of Sulfate Radical Reac-
tions in Aqueous Solution
0 1
1.0
10.0
Oxidative Degradation Constant, k^2(103) (Experimental)
Figure 8. Comparison of calculated and experimental value of k,z
acid degradation. In the presence
of Mn, thiosulfate decreases k-iz at
low levels of dissolved S(IV).
9. Among other transition metals, Co
and Ni increase k-\2 an order of
magnitude less efficiently than Fe,
while T\ appears to reduce ki2, and
T\, V, Cr, and Cu have no significant
effects.
10. Decarboxylation is the major deg-
radation pathway. In the presence
of 0.03 mM Mn, 1.3 mole carbon
dioxide is generated from every
mole of glutaric acid degradation
up to 30% conversion. At high
conversion (80%), as much as 49%
of the carbon of adipic acid is
degraded to carbon dioxide.
11. With 0.1 mM Mn at 80% conver-
sion, the other degradation prod-
ucts of adipic acid are smaller
dicarboxylic acids (4%), monocar-
boxylic acids (3%), hydroxycarbox-
ylic acids (5%), hydrocarbons (1%),
glutaric semialdehyde (7%), and
other carbonyl compounds (1 %).
\2. In the presence of high Mn (1.0
mM), the retained degradation
products of adipic acid are primarily
valeric and glutaric acids.
13. Hydroxyacids and sulfonated acids
degrade slower than simple dicar-
boxylic acids by a factor of three,
while maleic acid, an unsaturated
dicarboxylic acid, degrades an
order of magnitude faster. There-
fore, hydroxyacids and aldehyde-
acids, which are sulfonated under
FGD conditions, accumulate as
ultimate liquid-phase degradation
products.
Recommendations
1. The oxidation of S(IV) under FGD
conditions should be studied
further with emphasis on the
individual and combined effects of
Mn, Fe, halides, and free radical
scavengers to better understand
the mechanism. Dissolved S(IV)
and oxygen should be monitored
during the experiments to confirm
the effects of these two variables.
2. Organic acids and their deliberate
derivatives should be studied for
oxidative degradation to differen-
tiate electron transfer mechanism
and hydrogen abstraction. Decar-
boxylation and general degradation
can then be discriminated and
compared as a function of func-
tional group composition and cata-
lyst environment.
Reactant
HSO3
Succinate
Fumarate
Adipate
Fe2*
Mn2*
Br~
cr
OH~
HSOi
Estimated Literature
*C/W"1S"V k(M''S'')
>5.3x103
>2.5x5.3x103
7.1x10*
1.6xW7
I.OxlO7
8.6x103 9.9x103
1.8x10w
1.7x10a (3.5±0.4)xW*
1.3xW3
4.6x1 07
103-104
<10S
6.
The degradation product distribu-
tion should be studied as a function
of reaction time or conversion to
understand better the oxidative
degradation of organic acids.
The oxidative degradation experi-
ments should be conducted over a
broad pH range, 3 to 7, to observe
the effect of pH on the degradation
of organic acids.
Thiosulfate and halides, especially
iodide and bromide, should be
tested in combination with organic
acids in FGD scrubbing systems on
a pilot plant scale to investigate
possible beneficial effects. If oxida-
tion is inhibited, then unsaturated
acids such as maleic acid would
have makeup rates comparable to
DBA or adipic acid.
If high level of sulfite oxidation is
expected, formic and maleic acids
are inferior to other alternatives
such as DBA and adipic acid.
-------
Y. J. Lee and G. T. Rochelle are with the University of Texas, Austin. TX 78712.
Charles B. Sedman is the EPA Project Officer (see below).
The complete report, entitled "Ox/dative Degradation of Organic Acids
Conjugated with Sulfite Oxidation in Flue Gas Desullurizaton," (Order No.
PB 88-180 674/AS; Cost: $19.95. subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22J61
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
'U.S.Government Printing Office: 1988 - 548-158/67112
-------
C/) -n -.
10 ?2
00 I *
00 £ w
X »
2 c
oo £
8
O :
2 i
3
3 1
0)
-. <
0:
^
01
M
O>
00
CD
CT!
; ; i
------- |