Destruction of malodorous compounds using heterogeneous photocatalysis.

Article abstract of DOI:10.1021/es980404f

Malodorous sulfur-containing compounds (thiophene, methyl disulfide, trimethylene sulfide, and propylene sulfide; model compounds for industrial wastewater) were photocatalytically oxidized over TiO₂ in an annular plug-flow reactor. A MS online system monitored the formation of products and byproducts in real time. Mineralization of the sulfur-containing compounds was confirmed by mass balance of CO₂ and SO₄^2-. Dilute concentrations of trimethylene sulfide and propylene sulfide were completely mineralized. For thiophene and methyl disulfide, however, partial oxidation was observed, generating SO₂ and SO as byproducts. Sensory analysis showed that for trimethylene sulfide and propylene sulfide, odor intensity after TiO₂/UV treatment was below the olfactive threshold limit of the human panel. Both GC-FID and sensory analysis indicated that the photocatalytic process efficiently destroyed the malodorous compounds.

Full text of DOI:10.1021/es980404f

Environ. Sci. Technol. 1999, 33, 2788-2792
UV light has attracted interest due to its potential application
for the destruction of many pollutants (6, 7). Many of the
available studies on TiO / UV photocatalysis have been
centered on the destruction of volatile organic compounds
Destruction of Malodorous
Compounds Using Heterogeneous
Photocatalysis
2
(
VOCs) for atmosphere remediation (8-10). However, the
photocatalytic destruction of malodorous compounds has
been only slightly explored in the gas phase (11-13).
M A R I A C . C A N E L A ,
In this work, we provide preliminary results to help the
R O S A N A M . A L B E R I C I ,
R A Q U E L C . R . S O F I A ,
M A R C O S N . E B E R L I N , A N D
W I L S O N F . J A R D I M *
evaluation of possible application of the TiO / UV-vis
2
photocatalytic process for the destruction of malodorous
sulfur-containing compounds by monitoring online both
target compounds and byproducts using a mass spectrometry
system. As the final goal is to lower the compound concen-
tration to a level below the human odor threshold limits, the
efficiency of the process was also validated using sensory
analysis.
Instituto de Qu ´ı mica, Universidade Estadual de Campinas,
CP 6154, CEP 13083-970, Campinas, SP, Brazil
Photocatalytic oxidation of the sulfur-containing compounds,
trimethylene sulfide (C3H6S), propylene sulfide (C3H6S),
thiophene (C4H4S), and methyl disulfide (C2H6S2), was carried
out using an annular plug flow reactor with TiO2 in a
supported form. Formation of products and byproducts
was monitored in real time using a mass spectrometry online
system. Mineralization of the sulfur-containing compounds
Experimental Section
Titanium dioxide, which was supported in the photocatalytic
reactor, as described by Alberici and Jardim (8), was obtained
from Degussa (P-25) with an average particle diameter of 30
nm, a crystal structure of primarily anatase, and a surface
2
-1
area of 50 ( 15 m g (BET). The contaminated atmosphere
was generated by continuously vaporizing the liquid, in
synthetic air. Trim ethylene sulfide, propylene sulfide,
thiophene, and methyl disulfide (all provided by Aldrich)
were used as received in a range of concentration between
20 and 86 ppmv, in synthetic air. Concentration values were
chosen considering the background levels of these com-
pounds in the environment and their extremely low odor
threshold.
2-
was confirmed by mass balance of CO2 and SO4
.
Dilute contaminated atmospheres of trimethylene sulfide
and propylene sulfide were completely mineralized.
For thiophene and methyl disulfide, however, partial
oxidation was observed, generating sulfur dioxide (SO2)
and sulfur oxide (SO) as byproducts, which were confirmed
by parent ion MS/MS spectra as well as by chemical
ionization. Sensory analysis showed that for trimethylene
sulfide and propylene sulfide, odor intensity after TiO2/UV
treatment was below the olfactive threshold limit of the
panel.
The annular photoreactor consists of a glass cylinder (3.5
cm i.d. × 86 cm height) and a 30 W black-light lamp (Sankyo
Denki Japan-BLB) that serves as the inner surface of the
annulus. The internal wall of the glass tube was coated with
-
4
2
TiO
soaking/ drying coating method. A TiO
.3 µm was estimated by the Scanning Electron Micrograph
technique (SEM). The UV light intensity at the TiO coating
was 4.5 mW cm measured by a Cole Parmer radiometer at
65 nm. The photoreactor was fed with synthetic air
2
at a loading density of 9.5 × 10 g/ cm , using a simple
2
film thickness of ca.
Introduction
5
2
The problem with malodorous emissions from sewage and
industrial wastewater treatment plants has been largely
discussed because it is a nuisance in any neighborhood (1).
Sulfur-containing compounds such as mercaptans, organic
sulfides, disulfides, and hydrogen sulfide are mainly respon-
sible for obnoxious odors and have an extremely low odor
threshold. Malodorous compounds can be associated with
odors of different characteristics (fecal, rotten fish, rancid,
etc.), according to their functional groups, and are readily
detected by the human nose (2, 3). Most techniques of routine
analysis are, however, unable to detect odor-causing com-
pounds, hence, their identification is rather difficult even
when using the most sophisticated analytical techniques (4).
Therefore, sensory analysis is a very useful auxiliary and
powerful tool to evaluate hedonistic characteristics of waters
and atmospheres.
-
2
3
containing 21% of oxygen and 23% of relative humidity, since
water vapor and oxygen are required to maintain long-term
catalyst photoactivity (14-17). Experiments were performed
-
1
in a single pass mode at a flow rate of 250-500 mL min
,
which corresponds to a gas residence time of 1.61-0.81 min
at room temperature. No mass transfer limitations were
observed when this reactor was used in the destruction of
many VOCs, as reported in details by Alberici and Jardim (8).
Steady-state conditions were normally achieved after 30 min
of UV-irradiation. Conversion rates were monitored using a
GC-FID (SHIMADZU GC-14B gas chromatograph) equipped
with a DB-624 (30 m × 0.54 mm × 3 µm J&W) fused silica
megabore column. Experiments under UV irradiation, but
in the absence of catalyst, were performed to evaluate
conversions owing to photolysis only. A more detailed
discussion of the experimental apparatus and procedures is
described elsewhere (8).
To control odors, there are several well-established
conventional technologies, including adsorption by activated
carbon, biofiltration and bioscrubbing, wet chemical scrub-
bing, thermal oxidation, and prevention. Each of these
techniques displays a variety of advantages and disadvantages
and different degrees of cost-effectiveness (5). Recently,
The destruction of target compounds and the formation
of byproducts during the gas-phase photocatalytic oxidation
were monitored using a mass spectrometry online monitoring
system and selected ion monitoring (SIM). The catalytic
photoreactor outlet was connected directly to the gas inlet
of an Extrel (Pittsburgh, PA) pentaquadrupole mass spec-
2
heterogeneous photocatalysis using TiO as catalyst and near
*
Corresponding author phone/ fax: +55 19 7883135; e-mail:
wfjardim@iqm.unicamp.br.
2
7 8 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 16, 1999
10.1021/es980404f CCC: $18.00

1999 Am erican Chem ical Society
Published on Web 07/13/1999

FIGURE 1. Schematic representation of the photocatalytic degradation system and the MS/MS online monitoring: A, black-light lamp;
B, photoreactor; C, sulfur-containing compounds reservoir; D, flow meter; E, carrier gas; F, waste; G, needle valve flow control; and H,
mass spectrometer.
trometer (18, 19), consisting of three mass analyzers (Q1, Q3,
Q5) and two reaction quadrupoles (q2, q4). The gaseous
Results and Discussion
mixture was ionized by either 70 eV electron impact (EI) or
chemical ionization (CI). The pentaquadrupole mass spec-
trometer was also used to help identify products through
collision induced dissociation (CID) of mass-selected ions
in MS/ MS spectrometry. The mass spectrometer is also
equipped with a second inlet system, from which known
amounts of calibration gases were introduced to quantify
the compounds. This procedure was used for target com-
pounds only (Figure 1).
2
Initial experiments showed that both UV light and TiO were
necessary to the oxidation of the sulfur-containing com-
pounds. High levels of destruction of these compounds were
measured using both the GC-FID and the mass spectrometry
online monitoring system. Figure 2 displays the 70 eV mass
spectra obtained before and after UV-irradiation, showing
the disappearance of the target compound with concomitant
generation of new signals due to byproducts. Table 1 shows
further details for each peak with their respective identifica-
tion, based on the National Institute of Standard (NIST) mass
After the photooxidation experiments, the reactor was
washed six times with 50 mL of water to remove all adsorbed
products. The concentration of sulfate ions was determined
by turbidimetry (20). Sulfur dioxide was concentrated by
scrubbing through sodium tetrachloromercurate(II) and
subsequently determined by colorimetry using p-rosaniline
2
spectral library. In the TiO / UV degradation of propylene
sulfide (Figure 2a) and trimethylene sulfide (Figure 2b),
carbon dioxide was the only final product in the gas phase
identified by MS (m/ z 44). For thiophene (Figure 2c) and
methyl disulfide (Figure 2d), the ions of m/ z 64 and 48 are
also present in the respective mass spectra. The identities of
these ions were investigated using tandem mass spectrometry
(
21). Carbon dioxide was monitored in the gas phase using
flow injection analysis (FIA) (22). In this method, CO diffuses
through a PTFE membrane toward a deionized water stream,
2
(
MS/ MS) and CID (24). The ion m/ z 64 dissociates producing
the fragments of m/ z 48 and 32. Considering that m/ z 64
-
+
forming HCO
a conductometric detector. The solution conductance is
3
and H ions that continuously flow through
•+
•+
corresponds to SO
generated by O loss and the fragment of m/ z 32 (S ) by
2
, the fragment of m/ z 48 (SO ) is
•
+
proportional to the total CO
sample.
2
concentration of the gaseous
•
+
•+
2
either O loss from SO (25) or O
2
loss from SO
. Because
the fragment of m/ z 48 also dissociates to m/ z 32, it is not
Sensory analysis was carried out with eight panelists
trained to use Flavor Profile Analysis (FPA) (20). Panelists
were carefully selected according to their sensitivities to the
basic odor and had been trained to sample evaluation and
description according to both the Standard Methods for the
Examination of Water and Wastewater (20) and Dam a´ sio
and Costell (23).
possible to determine whether the ion of m/ z 48 results from
•+
dissociation of SO
2
(m/ z 64) or direct ionization of SO.
However, thiophene photodegradation experiments moni-
tored by chemical ionization mass spectrometry (Figure 3)
proved that the ionic fragment of m/ z 48 is formed directly
from neutral SO. Before UV-irradiation, only protonated
thiophene (m/ z 85) is observed in the CI mass spectrum
(
Figure 3a). After partial photodegradation (Figure 3b), signals
Panelists were exposed to the treated atmosphere after
the TiO / UV and the odor intensity was compared to the
2
+
corresponding to protonated SO
protonated SO (SOH ) of m/ z 49 are also detected. Note that
no dissociation of SO H to SOH is expected due to the
2
mild chemical ionization conditions and the highly unfavor-
able O loss from the closed-shell ion SO
disulfide however, EI-MS monitoring showed that only SO
is produced (spectra not shown).
2
(SO
2
H ) of m/ z 65 and
+
initial concentration of the compounds kept in a Tedlar bag.
The panelists used a flat scale of 10 cm to register the odor
intensity. The flat scale is a horizontal line with two
extremities, ranging from weak to strong odor intensity. The
panelists score the smell in the outlet reactor after comparing
the odor with odor free air and an air sample collected before
irradiation. Sensory analyses were carried out only for
compounds that were totally mineralized as determined by
+
+
+
2
H . For methyl
2
Figure 4 shows the MS-SIM profile of the online monitor-
ing of TiO / UV degradation of methyl disulfide (Figure 4a),
thiophene (Figure 4b), propylene sulfide (Figure 4c), and
trimethylene sulfide (Figure 4d). As the UV light is turned on,
the concentration of the sulfur-containing compounds
2
2
-
both mass balance (CO
2
and SO
4
) and MS analysis, to avoid
exposing the panelist to possible hazardous compounds
formed due to partial destruction of the parent compound.
VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 7 8 9

FIGURE 2. EI mass spectra (70 eV) of the gaseous synthetic air mixture containing (a) propylene sulfide, (b) trimethylene sulfide, (c)
thiophene, and (d) methyl disulfide compounds before and after 30 min of UV-irradiation. Experimental conditions: 30 W black-light lamp,
21% oxygen, and 23% relative humidity.
decreases rapidly with the concomitant generation of carbon
dioxide. The ions of m/ z 48 and 64 have also been used to
monitor the photodegradation of methyl disulfide (Figure
and this low sulfur yield can be attributed to SO formation
(not quantified), as demonstrated by the CI-MS analysis. For
methyl disulfide and thiophene, CO could not be quantified
2
•
+
4
a) and thiophene (Figure 4b). Note that in Figure 4a, SO
due to the interference of SO in the Conductometry-Flow
2
•+
is produced mainly from dissociation of the SO
in Figure 4b, SO corresponds both to dissociation of SO
and direct ionization of SO.
2
ion, whereas
Injection-Analysis (FIA). Sulfur dioxide also diffuses through
the PTFE, yielding conductance signals.
•
+
•+
2
Although there are very few papers on the literature dealing
with the destruction of sulfur-containing compounds using
the photocatalytic process, recently, Peral and Ollis (26) using
Table 2 summarizes the results obtained in the gas-phase
photocatalytic destruction of all four sulfur-containing
compounds under optimized conditions. High conversion
yields (>99%) were obtained for propylene sulfide and
TiO
of the dimethyl sulfide. The authors did not detect any sulfur
compound on the TiO , which was attributed to either the
2
/ UV process obtained 10% conversion in the destruction
trimethylene sulfide, with complete mineralization to CO
2
2
-
and SO
the photodegradation of methyl disulfide and thiophene,
SO was detected as an additional byproduct. For methyl
4
, as shown by carbon and sulfur mass balance. In
2
2 3
formation of SO or SO or to the formation of another
mercaptan. Suzuki (12), using a honeycomb type of gaseous
2
disulfide, 100% sulfur mass balance was achieved. For
thiophene, however, sulfur mass balance reached 88% only,
reactor, found that the destruction rates of S-compounds
(CH SH and H S) were in the range of 0.13 min .
3 2
-
1
2
7 9 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 16, 1999

TABLE 1. Major Ions in the 70 eV EI Mass Spectra of Sulfur
Containing Compounds
2
FIGURE 4. Diagrams of SIM-MS monitoring of TiO /UV degradation
of methyl disulfide (a), thiophene (b), propylene sulfide (c), and
•
+
trimethylene sulfide (d): (O) target compounds, (9) m/z 44 (CO
2
),
•
+
•+
2
(2) m/z 48 (SO ), and ([) m/z 64 (SO
). Dashed lines indicate the
time when the reactor was turned on.
SCHEME 1
2
FIGURE 3. Methane-CI mass spectra of TiO /UV degradation of
thiophene (a) before and (b) during UV-irradiation.
sulfur dioxide, and 0.12 mol of sulfur oxide were generated.
For 1 mol of methyl disulfide however, 1.53 mol of sulfate
ions and 0.47 mol of sulfur dioxide were generated.
The formation of different radicals from the illumination
The general equations for the complete photocatalytic
conversion of sulfur-containing compounds may be de-
scribed by the following stoichimetry
2
of TiO in water is well established in the literature (6). Both
C H S + (4x+6z+y)/ 4O f
x
y
z
2
positive holes and hydroxyl radicals have been proposed as
the oxidizing species responsible for initiating the attack on
organic solutes. In Scheme 1 we show a proposed mechanism
with the parallel reaction paths for both positive holes and
hydroxyl radicals. When photocatalytic oxidation is con-
ducted in the presence of water, the primary product of the
electron transfer is often an adsorbed hydroxyl radical (step
+
2-
xCO + 2zH + zSO4 + (y-z)/ 2H O (1)
2
2
and could be applied to trimethylene sulfide and propylene,
but for thiophene and methyl disulfide, the formation of SO
and SO as byproducts altered the reaction stoichimetry. For
mol of thiophene, 0.82 mol of sulfate ions, 0.06 mol of
2
1
TABLE 2. Initial Conditions and Sulfur-Containing Compounds Degradation Rates
conv oxid rate SO42 rate
-
SO2 rate
CO2 rate sulfur balance carbon balance
Cinlet res. time
flow
substrate
(ppmv) (min) (mL/min) (%) (µmol/min) (µmol/min) (µmol/min) (µmol/min)
(%)
(%)
trim ethylene sulfide
propylene sulfide
thiophene
61
86
54
34
1.81
0.99
1.30
0.85
262
409
324
474
99
99
99
99
0.72
1.58
0.79
0.72
0.71
1.58
0.65
0.55
nd
nd
0.05
0.17
2.16
4.65
a
99
100
88
100
98
a
m ethyl disulfide
a
100
a
a
2 2
CO analysis was not carried out due to SO interference in the Conductom etry-Flow Injection-Analysis (FIA).
VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 7 9 1

Acknowledgments
Financial support from the Funda c¸ a˜ o de Amparo a` Pesquisa
do Estado de S a˜ o Paulo (FAPESP) (Grants 97/ 01545-6, 97/
0
0758-6, 95/ 9497-5, and 97/ 6172-3) and the Conselho Na-
cional de Desenvolvimento Cientifico e Tecnol o´ gico (CNPq)
are greatly acknowledged.
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FIGURE 5. Performance of the TiO
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(
(
intermediates) because gas-phase reactions are very fast,
(
2
-
generating as final products SO
mass balance.
2
and SO
4
as shown by the
(
(
Although sulfate ions formed during the photocatalytic
process were adsorbed onto the catalyst surface, no catalytic
deactivation was observed in these experiments. Using H
Canela et al. (13) showed that sulfate ions adsorbed onto
TiO promoted a decrease in the photocatalytic activity when
2
S,
2
SO
.23 (section 2170, Flavor Profile Analysis), 4.126-4.132 (4500-
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(
(
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S (600 ppmv; corresponding to 5.4
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-
1
2-
2-
µmol min of SO
4
4
1
991, 43, 454-456.
-
1
µmol min of SO
2
(23) Dam a´ sio, M. H.; Costell, E. Rev. Agroqu ´ı m. Tecnol. Aliment. 1991,
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maintained for 24 h of continuous use. For all the sulfur-
(
containing compounds tested in the present work, the
-
1
maximum sulfate ions load observed was 1.58 µmol min
,
as shown in Table 2.
(
A comparative performance of the analytical technique
used to monitor the target compounds (GC-FID) versus the
sensory analysis is shown in Figure 5. Both GC-FID and
sensory analysis indicate that the photocatalytic process was
efficient to destroy the malodorous compounds. The panelists
were unable to detect odor for both methylene sulfide and
propylene sulfide in the outlet reactor, within 60 min after
of the irradiation. On the other hand, using chromatographic
analysis, after 15 min, it was not possible to detect the target
compound in the outlet reactor. This result emphasizes the
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4
Received for review April 20, 1998. Revised manuscript re-
ceived April 30, 1999. Accepted May 20, 1999.
ES980404F
2
7 9 2
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 16, 1999