Journal of Photochemistry and Photobiology A: Chemistry 156 (2003) 121–126
Kinetic investigation on UV and UV/H
2
O
2
degradations of
pharmaceutical intermediates in aqueous solution
Antonio Lopez
a,∗
, Anna Bozzi
a
, Giuseppe Mascolo
a
, John Kiwi
b
a
Department of Water Research and Technology, CNR-IRSA, Via F. de Blasio 5, Bari 70123, Italy
b
Laboratory of Photonics and Interfaces, Institute of Molecular Chemistry and Biology,
Swiss Federal Instituteof Technology, Lausanne 1015, Switzerland
Accepted 5 December 2002
Abstract
The degradation kinetics of two pharmaceutical intermediates (5-methyl-1,3,4-thiadiazole-2-methylthio (MMTD-Me) and 5-methyl-
1,3,4-thiadiazole-2-thiol (MMTD)) have been studied in order to assess the effectiveness and the feasibility of UV processes for the
decontamination of water polluted by such intermediates. Experiments were carried out, at 25
◦
C, treating, in a batch reactor, aqueous
solutions (1 and 100 mg/l) of both compounds by UV radiations (254 nm) in the presence or absence of hydrogen peroxide. For both
substrates, the results showed that: (i) no degradation occurred when H
2
O
2
alone was used; (ii) UV and UV/H
2
O
2
processes were both
effective for degrading the substrates; (iii) substrates degradation by photo-oxidation was always faster than by direct photolysis; (iv)
during direct photolysis, a lower substrate initial concentration lead to a faster and more efficient degradation. The quantum yields of the
photolytic process were experimentally measured for both substrates resulting 14.1 ± 1.5 and 12.0 ± 0.7 mmol einstein
-1
for MMTD-Me
and MMTD, respectively. Carrying out photo-oxidation experiments using excess of peroxide (i.e., initial substrate concentration of 1 mg/l
and H
2
O
2
/substrate molar ratios of 50/1, 42/1, 34/1 and 23/1), first- and second-order rate constants for MMTD-Me and MMTD degradation
were calculated. In particular, the values of these latter resulted (8.3 ± 0.8) × 10
8
and (1.6 ± 0.5) × 10
10
M
-1
s
-1
, respectively. Our results
show that to remove 99% of a few g/l of the pharmaceutical intermediates with a H
2
O
2
dose of 1 mg/l, 55 and 2.7 min for MMTD-Me
and MMTD are necessary, respectively.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Kinetics; Pharmaceutical intermediates; UV; UV/H
2
O
2
; Water treatment
1. Introduction
Waters contamination by pharmaceuticals is widely doc-
umented: estrogens, cholesterol-lowering drugs, pain re-
lievers, antibiotics, caffeine and anti-depressants have been
found in lakes, rivers and groundwater [1–4]. These com-
pounds reach waterways mainly through the discharge of
wastewaters both rough and treated. Additional pollution
sources are direct emissions from production sites, disposal
of surplus-drugs in households, excretion after applications
for human and animal medical care or therapeutic treatment
of livestock on field.
The conventional treatments carried out at wastewater
treatments plants (i.e., preliminary, primary and secondary)
usually do not effectively remove pharmaceutical deriva-
tives. Therefore, in order to meet the quality’s standards re-
quired for wastewaters discharge the effluents contaminated
∗
Corresponding author. Tel.: +39-80-5820506; fax: +39-80-5313365.
E-mail address: an.lopez@area.ba.cnr.it (A. Lopez).
by pharmaceutical derivatives must be pre- or post-treated by
appropriate physicochemical processes. Recently a growing
interest has been observed in the area of UV activated pro-
cesses due to [5]: (i) the continuous decrease of treatments
costs due to the breakthrough into the market of relatively
cheap low-energy UV lamps; (ii) the possibility to avoid, by
using non-contact reactors, the UV lamp fouling; (iii) the
simultaneous use of UV rays and chemical oxidants (e.g.,
ozone or hydrogen peroxide). Because of their specific tech-
nological requirements, UV based treatments are suitable
for removing organic pollutants from water or wastewater
with a low content of suspended solids and aromatic or-
ganic compounds due to the low light scattering and optical
absorption.
When UV light is absorbed directly by H
2
O
2
,
•
OH rad-
icals are generated by photolysis of the –O–O– peroxidic
bond (H
2
O
2
+ hν → 2
•
OH). Hydrogen peroxide absorbs
light (depending on its concentration) in the range of the
185–300 nm, the highest hydroxyl radical yields are obtained
when short-wave ultraviolet radiations (200–280 nm) are
1010-6030/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1010-6030(02)00435-5