Photocatalytic degradation kinetics and mechanism of environmental pharmaceuticals in aqueous suspension of TiO2: A case of sulfa drugs

Article abstract of DOI:10.1016/j.cattod.2010.02.068

The photocatalytic degradation kinetics of three sulfa pharmaceuticals has been investigated in TiO₂ aqueous suspension. The disappearance of these three compounds follows a pseudo-first-order kinetics according to the Langmuir-Hinshelwood (L-H) model. The effects of catalyst amount, initial pH value, and initial concentration of each substrate on the photocatalytic degradation rates were measured in detail. It was observed that the surface reaction on TiO₂ played an important role in the degradation of sulfa pharmaceuticals, and the further study of reactive oxygen species (ROSs) indicated that both photohole (h⁺) and especial hydroxyl radical (OH), were responsible for the major degradation of sulfa pharmaceuticals. The fates of the sulfur and nitrogen elements in various sulfa pharmaceuticals as well as total organic carbon (TOC) were examined following their photocatalytic transformation. The data showed that all three pharmaceuticals could be completely mineralized into CO₂, H₂O and inorganic ions within 240 min. These results indicated that many intermediates were produced during the photocatalytic transformation of sulfa pharmaceuticals process. Based on the identified intermediates, two tentative degradation pathways for the photocatalytic degradation of sulfa pharmaceuticals were proposed, for example hydroxylation addition to parent pharmaceuticals and the cleavage of S-N bond from the sulfaniline attacked by photohole.

Full text of DOI:10.1016/j.cattod.2010.02.068

Catalysis Today 153 (2010) 200–207
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Photocatalytic degradation kinetics and mechanism of environmental
pharmaceuticals in aqueous suspension of TiO : A case of sulfa drugs
2
Hai Yanga,b, Guiying Li , Taicheng An , Yanpeng Gaoa,b, Jiamo Fua
a
a,∗
a
State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Resources Utilization and Protection,
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, No. 511, Kehua Street, Tianhe District, Guangzhou 510640, China
b
Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 30 March 2010
The photocatalytic degradation kinetics of three sulfa pharmaceuticals has been investigated in TiO2
aqueous suspension. The disappearance of these three compounds follows a pseudo-first-order kinetics
according to the Langmuir–Hinshelwood (L–H) model. The effects of catalyst amount, initial pH value,
and initial concentration of each substrate on the photocatalytic degradation rates were measured in
detail. It was observed that the surface reaction on TiO2 played an important role in the degradation
of sulfa pharmaceuticals, and the further study of reactive oxygen species (ROSs) indicated that both
photohole (h ) and especial hydroxyl radical ( OH), were responsible for the major degradation of sulfa
pharmaceuticals. The fates of the sulfur and nitrogen elements in various sulfa pharmaceuticals as well
as total organic carbon (TOC) were examined following their photocatalytic transformation. The data
showed that all three pharmaceuticals could be completely mineralized into CO2, H2O and inorganic
ions within 240 min. These results indicated that many intermediates were produced during the pho-
tocatalytic transformation of sulfa pharmaceuticals process. Based on the identified intermediates, two
tentative degradation pathways for the photocatalytic degradation of sulfa pharmaceuticals were pro-
posed, for example hydroxylation addition to parent pharmaceuticals and the cleavage of S–N bond from
the sulfaniline attacked by photohole.
Keywords:
Sulfa pharmaceutical
Photocatalytic
Kinetics
Mechanism
Reactive oxygen specie
+
•
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
organisms of the food chain [24,25], which may adversely affect the
environmental ecosystems [26] and human health [27]. Although
these kinds of sulfa pharmaceuticals have been used for several
decades, it is surprised that there are only few studies to investigate
their environmental and health effect. Thus, it is very essential to
study the transformation kinetics and mechanism involved in var-
ious environmental conditions. By doing this, their environmental
fate, transfer, effect and potential risk of these kinds of sulfa phar-
maceuticals can be properly elucidated in environmental waters as
well as during water treatment processes.
The presence of pharmaceuticals and personal care products
(
PPCPs) in surface water is an emerging environmental issue and
provides a new challenge to drinking water, wastewater, and water
reuse treatment system [1–4]. One class of antibiotics pharmaceu-
ticals, sulfa drugs, is frequently found in the environmental waters,
such as sewage treatment plants water [5–7] and river [8]. Several
recent researches also demonstrated the potential omnipresence
of sulfa pharmaceuticals in the soil environment [9] and manure
[
10–13]. It is because sulfa pharmaceuticals are often used in
Advanced oxidation processes (AOPs) are accepted as a valid
alternative to transform and decontaminate these soluble bio-
refractory human antibiotics. These processes named AOPs all rely
aquaculture [14], agriculture as herbicides and veterinary pharma-
ceuticals [15,16], and human beings for the treatment of respiratory
and urinary tract infections [17]. However, due to their high resis-
tance to the photodegradation [17–20] and biodegradation [21,22],
these kinds of pharmaceuticals were often excreted into sewage
with metabolites as well as the unchanged parent compounds after
usage [23]. These discharged sulfa pharmaceuticals could persist in
environmental waters for a long time and accumulate in various
•
on the generation of highly reactive OH radical as the main oxida-
tive species for the potential destruction and conversion of organic
pollutants into harmless substances [28–31]. Heterogeneous pho-
tocatalysis is one typical example of AOPs for the degradation of
pharmaceuticals and other organic pollutants in water [32–35].
In recent years, although there are already a few researches to
report the photocatalytic degradation possibility of sulfa pharma-
ceuticals, these works are still not so sufficient to fully understand
the transformation kinetics and mechanism of these kinds of phar-
maceuticals in water [36,37]. Thus the relationship between the
∗ Corresponding author. Tel.: +86 20 85291501; fax: +86 20 85290706.
E-mail address: antc99@gig.ac.cn (T. An).
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.068

H. Yang et al. / Catalysis Today 153 (2010) 200–207
201
reaction solution (3 mL) were obtained at fixed time intervals, fil-
tered through 0.2 ␮m Millipore filters and analyzed by HPLC and
HPLC/MS/MS. All experiments were carried out at room temper-
ature. The kinetic data are presented as means from triplicate
experiments, and the errors are below 5%.
2.3. Analytical procedures
Photocatalytic degradation kinetics of sulfa pharmaceuticals
was carried out by using an Agilent 1200 series HPLC under the
following conditions: Kromasil C18 column, 250 mm × 4.6 mm i.d.,
◦
performed at 30 C. The mobile phase was 30% CH CN and 70%
3
formic acid solution (0.3%, v:v) which was filtered with a Water
Associates (Milford, MA, USA) 0.45 ␮M filter. The flow rate of the
mobile phase was set as 1 mL/min.
A Dionex instrument equipped with a conductimeter detec-
tor has been employed to analyze the concentration of produced
anions and cations. The determination of ammonium ions has
been performed by adopting a column CS12A and 25 mM H SO4
as eluent, with a flow rate of 1 mL/min. The retention time is
2
Fig. 1. Molecular structure of three investigated sulfa pharmaceuticals.
5
.2 min for ammonium ions. The anions have been analyzed by
similar structure of different sulfa pharmaceuticals and the photo-
catalytic degradation activity towards different functional groups
is urgent to be investigated.
using AS9HC anionic column. The mixture of NaHCO (4.5 mM) and
K CO (0.8 mM) was used as eluent with a flow rate of 1 mL/min.
The retention times are obtained as 11.7 min and 12.8 min for
nitrate and sulphate, respectively. Total organic carbon (TOC) was
measured after filtered suspensions using a Shimadzu TOC-5000
3
2
3
In order to probe the environmental transformation characteris-
tic and the general degradation law of these sulfa pharmaceuticals,
the photocatalytic degradation kinetics and mechanism were
carried out contrastively, by chosen three different sulfa phar-
maceuticals, such as sulfachlorpyridazine, sulfapyridine and
sulfisoxazole, as model compounds. The photocatalytic degrada-
tion kinetics of three sulfa pharmaceuticals were compared with
different catalyst concentrations, initial pH values and initial sub-
strate concentrations. In addition, the contribution of different
◦
analyzer (catalytic oxidation on Pt at 680 C).
Degradation intermediates were analyzed using a HPLC/MS/MS,
a Shimadazu high performance liquid chromatography (HPLC) sys-
tem with a Kromasil C18 column (250 mm × 4.6 mm i.d.), SIL-HT
autosampler, LC-10 AT vacuum pump and API 3000 mass analyzer.
HPLC separations were performed at 0.5 mL/min with linear gradi-
ent elution as follows: from 90% A (5 mM formic acid solution) and
•
reactive oxygen species (ROSs), such as OH and photohole, to the
1
0% B (CH OH) to 40% A and 60% B within 40 min. An electrospray
3
photocatalytic degradation of sulfa pharmaceuticals is also exam-
ined in detail by using different specific scavengers. At last, the
general photocatalytic degradation mechanism of sulfa pharma-
ceuticals was also tentatively attempted based on the identified
intermediates and proposed degradation pathways.
interface (ESI) was used for the MS and MS–MS measurements in
positive ionization mode and full scan acquisition between m/z 100
and 350. The collision energy varied according to the requirement
of the different measurements, and the other parameters were set
as follows: the ESI was 5.5 keV, the source block and desolvation
◦
◦
temperature were 130 C and 400 C, respectively, the desolvation
2
. Materials and methods
.1. Chemicals and reagents
Sulfachlorpyridazine,
and nebulizer gas (N ) flow rate were set as 6 L/min and argon was
2
used as a collision gas at 2500 mbar.
2
3. Results and discussion
sulfapyridine
and
sulfisoxazole
(
Sigma–Aldrich) were used as received (≥99% purity, the structures
3
.1. The photocatalytic degradation kinetics of sulfa
are shown in Fig. 1). HPLC grade water was obtained from Millipore
Milli-Q System (Water, Millipore), which was treated by constant
illumination with a Xe arc lamp at 172 nm to keep total organic
carbon concentration below 13 ␮g/L. Acetonitrile and methanol
pharmaceuticals
The photocatalytic degradation kinetics of three sulfa pharma-
ceuticals was investigated, and the results are shown in Fig. 2. After
6
8
(
HPLC grade) were purchased from Sigma.
0 min of illumination, the removal efficiencies were achieved as
5.2%, 92.5% and 85.0% for sulfachlorpyridazine, sulfapyridine and
2
.2. Irradiation procedures
sulfisoxazole, respectively, with an initial concentration of 100 ␮M,
a TiO concentration of 2.0 g/L and an initial pH value 7.0. The pho-
tocatalytic degradation of three sulfa pharmaceuticals can be well
described by Langmuir–Hinshelwood (L–H) model [38]:
2
The adsorption and photocatalytic degradation of sulfa phar-
maceuticals were carried out in a Pyrex reactor (150 mL) with a
double-walled cooling-water jacket to keep the constant tempera-
ture of solutions throughout the experiments. The light source was
a high-pressure mercury lamp (GGZ-125, Shanghai Yaming Light-
ing, Emax = 365 nm) with a power consumption of 125 W, housed
in one side of the photocatalytic reactor to provide the irradiation.
Prior to illumination, a suspension of 150 mL sulfa pharmaceuti-
cals (100 ␮M) adding different concentrations of the photocatalyst
dc
dt
kKC
−
=
(1)
1 + KC
When concentration is very low (i.e. KC ꢀ 1), Eq. (1) simplifies to a
pseudo-first-order kinetic law:
dc
−
= k C
(2)
1
dt
(
Degussa P25) was stirred in the dark for 30 min to achieve the
adsorption–desorption equilibrium. Then, the UV light was turned
on for the photocatalytic degradation experiments. Samples of the
where k is the pseudo-first-order rate constant. The rate constants,
1
the linear plots of −ln(C/C ) vs. time, shown in the inset of Fig. 2,
0

2
02
H. Yang et al. / Catalysis Today 153 (2010) 200–207
Fig. 4. Effect of pH values on the photocatalytic degradation rate constants of sul-
fachlorpyridazine (ꢀ), sulfapyridine (᭹), and sulfisoxazole (ꢁ) with 100 ␮M and
Fig. 2. Photocatalytic disappearances of sulfachlorpyridazine (ꢀ), sulfapyridine (᭹),
and sulfisoxazole (ꢁ) with 100 ␮M, 2.0 g/L TiO2 and pH value 7.0. Inset: The lin-
ear transformation of −ln(C/C0) vs. time, for the photocatalytic degradations of
sulfachlorpyridazine (ꢀ), sulfapyridine (᭹), and sulfisoxazole (ꢁ).
2.0 g/L TiO2 concentration.
linearly with TiO2 concentration ranged from 0.25 g/L to 3.0 g/L.
These increases in the rate constants seem to be due to the increase
in the total surface area of photocatalysts, namely number of active
sites, available for the photocatalytic reaction as the dosage of pho-
tocatalyst increased [39]. These results suggest that the similar
structures of these three sulfa pharmaceuticals have the similar
change trends of degradation rates in the catalyst concentration
range of 0.25–3.0 g/L.
were calculated as 0.031 min−1, 0.043 min and 0.031 min for
sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respectively.
So it can be seen that sulfachlorpyridazine and sulfisoxazole have
the same degradation rates, while sulfapyridine degraded more
quickly than the other two compounds.
−1
−1
3.2. Effect of the catalyst concentration
3.3. Effect of the initial pH value
To optimize the TiO catalyst concentration, the effect of catalyst
2
These three sulfa pharmaceuticals are all zwitter ionic com-
amount on the photocatalytic degradation rate of sulfa pharma-
ceuticals was carried out. Fig. 3 shows the dependence of TiO2
concentrations on the degradation rate constants. It was observed
that all the rate constants increased with the increase of the
amount of TiO2 catalyst for these three sulfa pharmaceuticals.
pounds, generally existing as three different ionic forms in water,
which mainly depend on the pH value of the solution. The first
dissociation constants (pKa,1) were measured to be 2.00 ± 3.00,
4
.25 ± 0.30 and 1.50 ± 0.30, and the corresponding second dissoci-
ation constants (pK ) were 5.90 ± 0.30, 8.43 ± 0.03 and 5.00 ± 0.07,
−
1
2
The rate constants increased from 0.020, 0.029 and 0.027 min
for sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respec-
tively [17,18,40]. Thus the initial pH value can change the existed
forms of sulfa pharmaceuticals in water, and thus significantly
influence the photocatalytic degradation rates. Fig. 4 depicts the
relationship between the rate constants with the initial pH val-
ues of three sulfa pharmaceuticals solution. From this figure, it
can be seen that these three compounds have different change
trends in the pH values range of 3.0–11.0. For sulfachlorpyridazine,
−1
at 0.25 g/L TiO to 0.032, 0.043 and 0.033 min at 3.0 g/L TiO for
2
2
sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respectively.
For sulfachlorpyridazine and sulfapyridine, the rate constants
increased dramatically as TiO concentration is lower than 2.0 g/L,
2
and then rose very slightly with further increase of catalyst con-
centration. While for sulfisoxazole, the rate constants increased
−
1
the rate constant decreased very dramatically from 0.060 min
−
1
at pH value 3.0 to 0.031 min at pH value 7.0 at first, and then
−
1
increased to 0.042 min with further increasing the pH value up
to 11.0. For sulfisoxazole, the curve of rate constants plot against pH
value was much similar as that of the sulfachlorpyridazine. That is,
−
1
the rate constants decreased at first from 0.049 min at pH value
−
1
3
.0 to reach the bottom 0.031 min
at pH value 9.0, and then
increased very slightly with the further increase of the pH value.
While as for sulfapyridine, the rate constants exhibited totally dif-
ferent trend from the two former, and the rate constants increased
−
1
−1
almost linearly from 0.026 min at pH value 3.0 to 0.052 min at
−
1
pH value 9.0, and then decrease to 0.050 min at pH value 11.0.
From the above results, it is found that the initial pH value has dif-
ferent influences on the photocatalytic degradation rate constants
although these three sulfa pharmaceuticals have similar sulfaniline
structure. High rate constants in low pH value range for sulfachlor-
pyridazine (pKa,1 = 2 ± 3) and sulfisoxazole (pK = 1.5 ± 0.3) are
a,1
because that they are difficult to be protonated in weak acidic
solution (pH value ≥ 3.0), these neutral structure can be adsorbed
onto the positively charged TiO2 surface below the zero point
Fig. 3. Effect of TiO2 dosage on the photocatalytic degradation rate constants of
sulfachlorpyridazine (ꢀ), sulfapyridine (᭹), and sulfisoxazole (ꢁ) with 100 ␮M and
pH value 7.0.

H. Yang et al. / Catalysis Today 153 (2010) 200–207
203
Fig. 6. Effects of initial concentration of sulfa pharmaceuticals on the photocat-
alytic degradation rate constants for sulfachlorpyridazine (ꢀ), sulfapyridine (᭹),
and sulfisoxazole (ꢁ) with 2.0 g/L TiO2 concentration and pH value 7.0. Inset: The
relationship between 1/k1 and the initial concentration of sulfachlorpyridazine (ꢀ),
sulfapyridine (᭹), and sulfisoxazole (ꢁ) with 2.0 g/L TiO2 concentration and pH value
7.0.
various initial pH values. It was found that the acidic media were
prone to the adsorption of sulfachlorpyridazine and sulfisoxazole
onto TiO , while the adsorption of sulfapyridine was more efficient
2
in neutral and alkaline media. These findings can be well confirmed
by the conclusion that the photocatalytic degradation rates with
different initial pH value were greatly controlled by the existed
forms of the sulfa pharmaceuticals in the solution, which deter-
mined the adsorption amounts of sulfa pharmaceuticals onto TiO2
in the solution.
3.4. Effect of the initial concentration
The dependence of the degradation rate constants on the ini-
tial concentrations of the substrate was also investigated and the
results are shown in Fig. 6. It can be seen that the rate constants of all
the three sulfa pharmaceuticals decrease with the increase of ini-
Fig. 5. The normalized sulfachlorpyridazine (a), sulfapyridine (b) and sulfisoxazole
c) concentration vs. adsorption time at various pH value with 100 ␮M and 2.0 g/L
TiO2 concentration.
(
−
1
tial concentration, from 0.065, 0.095 and 0.056 min at 50 ␮M to
−
1
0
.016, 0.020, and 0.018 min at 200 ␮M for sulfachlorpyridazine,
(
pH value 6.3). However, the sulfapyridine (pKa,1 = 4.25 ± 0.3) is
sulfapyridine and sulfisoxazole, respectively. With 60 min illumi-
nation, all three sulfa pharmaceuticals with an initial concentration
of 50 ␮M can be completely photocatalytically degraded, whereas
at an initial concentration of 200 ␮M, only less than 70% of sulfa
pharmaceuticals were destroyed (The data are not shown). Lower
rate constants at higher initial concentration may be attributed to
prone to be protonated in the same low pH value range, thus
the electrostatic repulsion between the protonated sulfapyridine
and the positive TiO2 greatly retards their adsorption and subse-
quently results in the low photocatalytic degradation rate. On the
other hand, with further increase of the pH value, sulfachlorpyri-
dazine (pKa,2 = 5.90 ± 0.30) and sulfisoxazole (pK = 5.00 ± 0.07)
a,2
the fact that the more substrates can occupy more TiO active sites,
2
will lose a proton and exist in anionic forms. Hence, these two
negative sulfa pharmaceutical molecules cannot be easily adsorbed
onto the surface of TiO2 with the same negative charges (pH
value ≥ 6.3). Nevertheless, sulfapyridine (pKa,2 = 8.43 ± 0.03) is a
neutral molecule when the pH value is lower than 8.43 and thus the
adsorption of sulfapyridine onto TiO2 becomes much easier than
the other two negative sulfa pharmaceuticals. Consequently, it is
not surprised that low degradation rates for sulfachlorpyridazine
and sulfisoxazole and high degradation rate for sulfapyridine were
observed in weak alkaline solution as shown in Fig. 4.
which subsequently suppress generation of the oxidants. Further-
more, the higher sulfa pharmaceutical concentrations absorb more
photons, so the shortage of photons to activate TiO2 inhibited the
degradation of sulfa pharmaceuticals at a higher initial concentra-
tion [41].
Additionally, L–H equation also can be successfully used to
describe the relationship between the photocatalytic degradation
rate and the initial concentration of organic pollutant in heteroge-
neous photocatalytic degradation [36]. Through Eqs. (1) and (2), Eq.
(3):
In order to further validate the proposed conclusion that the
photocatalytic degradation rate at the different pH values is
significantly affected by the adsorption performance of sulfa phar-
kK
k1 = 1
(3)
+ KC
maceuticals onto TiO surface, the adsorption kinetics of three sulfa
2
pharmaceuticals was also carried out in detail. Fig. 5 shows the
adsorption of sulfa pharmaceuticals against the adsorption time at
was obtained and transformed into its inverse function results in a
linear relationship with an intercept of k and a slope k K , as
−
1
−1 −1

2
04
H. Yang et al. / Catalysis Today 153 (2010) 200–207
Table 1
−1
−1
to 0.010, 0.012 and 0.009 min
0
.031, 0.043 and 0.031 min
Scavengers used, oxidizing species quenched, and k1 for sulfa pharmaceuticals after
quenched by scavengers.
for sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respec-
tively. The degradation rate of these three sulfa pharmaceuticals
with 67.7%, 72.1% and 71.0% is contributed by the OH radicals.
k1 (min−1
)
R2
•
Sulfa pharmaceuticals Scavengers
ROSs quenched
–
Sulfachlorpyridazine
Sulfapyridine
No scavengers
Isopropanol
KI
0.031
0.010
0.97
0.99
0.98
Comparatively, the rate constants also decreased very significantly
•
−1
−1
−1
OH
to 0.002 min , 0.004 min and 0.003 min after addition of KI
scavengers in the reaction solution. These results indicated that
93.5%, 90.7% and 90.3% of the degradation rate of these three sulfa
pharmaceuticals were originated from both the OH radicals and
photoholes. Thus the contribution percentage of photoholes in the
degradation rate was deduced as 25.8%, 18.6% and 19.3% by sub-
+
•
h /adsorbed OH 0.002
No scavengers
Isopropanol
KI
–
0.043
0.012
0.99
0.97
0.99
•
•
OH
+
•
h /adsorbed OH 0.004
Sulfisoxazole
No scavengers
Isopropanol
KI
–
0.031
0.009
0.98
0.97
0.98
•
OH
•
tracting the percentage of OH radicals from the total percentage,
+
•
h /adsorbed OH 0.003
for sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respec-
tively. Only 6.5%, 9.3% and 9.7% of the degradation rates were
from other ROSs, for sulfachlorpyridazine, sulfapyridine and sul-
fisoxazole, respectively. Therefore, it can be concluded that the
OH radicals are the predominant ROSs although both OH and h
together are responsible for the major degradation of sulfa phar-
Eq. (4) [42,43]:
1
k1
C
k
1
kK
•
•
+
=
+
(4)
where k1 is the pseudo-first-order rate constant (min 1), k is the
−
1
•
•
−
maceuticals. While the other ROSs (H O , O , HO2 and O2 )
2
2
2
intrinsic reaction rate constant (␮M/min), and K is the L–H adsorp-
together only play a very minor role in the degradation of three
sulfa pharmaceuticals.
−
1
tion constant of sulfa pharmaceuticals over TiO surface (␮M ) in
2
aqueous solution. Three satisfactory linear correlation coefficients
were obtained as 0.994, 0.992, and 0.984 between 1/k and the con-
1
centration ofsulfa pharmaceuticals givenin the insert of Fig. 6. From
the slope, corresponding to 1/k, and the y-intercept, correspond-
ing to 1/kK, the intrinsic reaction rate constant k were got as 3.13,
3.6. The preliminary reaction mechanism of sulfa
pharmaceuticals
3
.72 and 4.07 ␮M/min, and L–H adsorption constants K were also
The disappearances of three selected sulfa pharmaceuticals as a
function of the photocatalytic degradation time, together with evo-
−1
−1
−1
−2
−1
obtained as 2.40 ␮M , 1.02 × 10 ␮M
and 3.56 × 10 ␮M
lution of decontaminated products, such as NO3 , NH4+ and SO4
−
2−
,
for sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respec-
tively. It demonstrates that although the adsorption constants are
at different order of magnitude, the intrinsic reaction rate constants
are all at the same order of magnitude and with much closed values.
A conclusion can be drawn that the degradation of all these three
sulfa pharmaceuticals occurred mainly on the surface of TiO2 by
are depicted in Fig. 7. After 60 min irradiation, 85.2%, 92.5% and
85.0% of sulfachlorpyridazine, sulfapyridine and sulfisoxazole are
degraded, while 81.3%, 90.3% and 81.0% of sulfur atoms are con-
verted into SO42 , respectively. These results indicated that the
cracking of the S–N bond from sulfa pharmaceuticals was a main
initial degradation pathway. In addition, the decontamination of
nitrogen atom is also found as an important pathway. However,
−
•
the oxidation reaction, such as photohole and OH radical, although
their adsorption was so significantly different in neutral solution.
Hence, the contribution of different ROSs to the photocatalytic
degradation is needed to be further investigated.
for the conversion of nitrogen atom, the release of NO3 and NH4+
are significantly dependent on the structure of sulfa pharmaceuti-
cals. As for sulfachlorpyridazine, only 17.2% nitrogen are released
−
as NO3 (2.1%) and NH4+ (15.1%) in the solution, while the rel-
−
3.5. The contribution of different ROSs
−
ative high conversion efficiencies of 70.5% (5.1% NO
and 65.4%
3
NH4+) and 74.4% (5.1% NO3 and 69.4% NH4+) are obtained for
sulfapyridine and sulfisoxazole, respectively. Low conversion effi-
ciencies of nitrogen atoms in sulfachlorpyridazine may be due to
that the azo group in pyridazine ring is likely to be released as
N2 [46]. Ammonia ion is predominant specie of inorganic nitro-
gen released from degradation of all three sulfa pharmaceuticals.
The percentages of NH4+ from the total inorganic nitrogen are
obtained as 87.8%, 92.8% and 93.3% for sulfachlorpyridazine, sul-
fapyridine and sulfisoxazole, respectively. It is because that the
nitrogen in heterocyclic aromatic rings can be transformed to both
−
During photocatalytic degradation reaction, the generation of
photoelectrons and photoholes pairs is the beginning of all the oxi-
dation processes. Thus a series of ROSs, such as OH, O , HO
and H O , are subsequently produced from primary active pho-
togenerated holes and electrons [44]. In order to distinguish the
•
•
− •
2
2
2
2
•
contribution of the surface reaction with photohole or OH rad-
ical from other ROSs on the photocatalytic degradation of sulfa
pharmaceuticals, different scavengers were employed to investi-
gate their effects on the photocatalytic degradation kinetics. In this
paper, 0.1 M isopropanol was added in the reaction solution as scav-
engers of OH radicals [2], and potassium iodine (KI) was selected
as scavengers of both OH radicals and photoholes [44,45]. It is
•
NO3 and NH4+ species, while secondary, tertiary and quaternary
nitrogen atoms are photo-converted predominantly to ammonia
[36]. From above-mentioned results, we can infer that all three
sulfa pharmaceuticals can be partly photocatalytic decontaminated
into harmless inorganic compounds via the production of a series of
degraded intermediates. In order to further confirm this conclusion,
TOC concentrations during the photocatalytic degradation of these
three sulfa pharmaceuticals were measured, and the decrease pro-
files were also shown in Fig. 8. From the figure, it can be easily found
that all these three pharmaceuticals can be almost mineralized into
CO2 with TOC decrease efficiencies of 90.8%, 88.8% and 81.5%, for
sulfachlorpyridazine, sulfapyridine and sulfisoxazole, respectively.
Thus HPLC and HPLC/MS/MS were used to separate and identify
the produced intermediates during the photocatalytic degrada-
tion. The structural assignments of all detected intermediates
−
•
•
because isopropanol can easily react with OH radicals converting
into relatively inert isoproanol radical with a high bimolecular rate
9
−1 −1
S [2]. On the other hand, a commonly
constant of 1.9 × 10 M
−
used electron donor, I ions, was selected to scavenge the photo-
•
holes and resulted OH radicals by forming relatively inert iodine
radicals [44]. Thus the photocatalytic reaction can be partly sup-
pressed with the addition of different scavengers in the reaction
solution. The obtained pseudo-first-order rate constants with or
without the addition of various scavengers and the corresponding
regression coefficients are all presented in Table 1.
From Table 1, it was observed that the degradations of three
sulfa pharmaceuticals were all suppressed in the presence of iso-
propanol. The pseudo-first-order rate constants decreased from

H. Yang et al. / Catalysis Today 153 (2010) 200–207
205
Fig. 7. The disappearance of sulfa pharmaceuticals and the evolution of nitrate,
ammonium and sulphate ions during photocatalytic degradation of sulfachlorpyri-
dazine (a), sulfapyridine (b) and sulfisoxazole (c) with 2.0 g/L TiO2 and pH value
Fig. 9. The evolution of intermediates formed during the photocatalytic degradation
of sulfachlorpyridazine (a), sulfisoxazole (b) and sulfapyridine (c) with 2.0 g/L TiO2
and pH value 7.0 as a function of irradiation time.
7
.0.
were based on both the analysis of the molecular ions peaks
and their corresponding fragmentation pattern. The similar evo-
lution profiles of the produced intermediates were obtained for
three sulfa pharmaceuticals, and the detailed curves are shown
in Fig. 9. From the figure, it can be seen that all intermediates
were formed very fast, reached the maximum peak at 15 min
or 20 min and then slowly disappeared. For sulfachlorpyridazine,
three obtained earlier intermediates with m/z = 302, correspond-
ing to the addition of 16 mass units to the parent compound, can be
attributed to monohydroxylated intermediates. At the same time,
two daughter intermediates with m/z = 318 were also detected,
which are corresponding to dihydroxylated derivatives of sul-
fachlorpyridazine. One more identified intermediate with m/z = 130
can be assigned to 6-chloropyridazin-3-amine, which was pro-
duced from the cleavage of S–N bond. Its corresponding further
oxidative intermediate with m/z = 112, 6-aminopyridazin-3-ol, was
also detected (shown in Fig. 9a). As for sulfisoxazole, the simi-
lar degradation intermediates were also found (shown in Fig. 9b).
Similar to sulfachlorpyridazine, two monohydroxylated interme-
diates with m/z = 284 and two dihydroxylated intermediates with
m/z = 300 were also found. The intermediate with m/z = 114, 3,4-
dimethylisoxazol-5-amine, produced by the cracking of S–N bond
Fig. 8. The decrease of TOC concentration for 100 ␮M sulfachlorpyridazine (ꢀ),
sulfapyridine (᭹) and sulfisoxazole (ꢁ) with 2.0 g/L TiO2 and pH value 7.0.

2
06
H. Yang et al. / Catalysis Today 153 (2010) 200–207
minor role during this process. For all sulfa pharmaceuticals exam-
ined, more than 81.0% of the sulphurs are photo-converted into
2
−
+
SO4 , and the nitrogens are converted predominantly into NH4
−
and a less extent into NO3 . It must be note that, for sulfachlor-
pyridazine, only a very small percentage of NH4+ was produced
as compared with the other two compounds. Variations in these
products during the transformation of sulfa pharmaceuticals are
dependent on the substrates molecular structure. At last, a general
photocatalytic degradation mechanism was tentatively proposed,
and the hydroxylation addition reaction and the cracking of S–N
bond resulted by photohole are considered as two predominant
routes for the degradation of sulfa pharmaceuticals.
Acknowledgments
Fig. 10. The proposed pathways of photocatalytic degradation of sulfa pharmaceu-
ticals.
This is contribution No. IS-1172 from GIGCAS. The authors
appreciate the financial supports from National Nature Science
Foundation of China (No. 40973068), the Earmarked Fund of the
State Key Laboratory of Organic Geochemistry (SKLOG2009A02)
and Knowledge Innovation Program of Chinese Academy of Sci-
ences (No. KZCX2-YW-QN103).
and lost sulfaniline, and its corresponding further oxidative inter-
mediate with m/z = 113, 3, 4-dimethylisoxazol-5-ol, were also
found. For sulfapyridine (shown in Fig. 9c), two monohydroxylated
intermediates with m/z = 266 were identified, while the dihydroxy-
lated intermediates were not detected. The daughter intermediate
with m/z = 95, 2-amine-pyridin, originated from the cleavage of S–N
bond and lost sulfaniline, was also found for this compound.
By comparison of the intermediates produced during the
photocatalytic degradation of three sulfa pharmaceuticals and con-
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