A. Bianco Prevot et al.
Catalysis Today xxx (xxxx) xxx–xxx
2
. Materials and methods
2.1. Chemicals
Melamine (2,4,6-triamino-1,3,5-triazine, 1), Fluka, Steinheim, Germany,
and cyanuric acid (2,4,6-trihydroxy-1,3,5-triazine, 4), Merck, Darmstadt,
Germany, were used as received. Ammeline (2,4-Diamino-6-hydroxy-1,3,5-
triazine, 2) and ammelide (2-amino-4,6-dihydroxy-1,3,5-triazine, 3) were
previously prepared [22]. Didodecyldimethylammonium bromide
(
DDDMAB), Eastman Kodak Co., Rochester, NY, and sodium dodecylsul-
phate (SDS), Darmstadt, Germany, were used as received. All other reagents
were at least analytical grade (Merck or Fluka). Pure water was provided by
−1
an UHQ II System (Millipore, > 18 mΩ cm , < 20 ppb organic carbon).
Ar and synthetic air were of 99.99% quality (Messer, Sulzbach, Germany).
Scheme 1. Chemical structures of the investigated substrates.
•
2
2
(8b)
(8c)
2.2. Irradiation experiments
•
2
•
2
2.2.1. Light source
O
2
A cylindrical Xe-excimer radiation source (length: 25 cm, external
diameter: 3 cm [23]) emitting at 172 nm ± 14 nm was operated at
In a previous work the VUV-technique was used to degrade atrazine
1
00 W and 175 ( ± 10%) kHz, using a high frequency, high voltage
[
11], a widely used, highly persistent herbicide and potential carcinogenic
power supply (ENI, Model HPG-2).
[
12,13]; following this approach aqueous solutions of atrazine yielded a
definitely lower amount of cyanuric acid, the fully hydroxylated s-triazine
derivative (4, Scheme 1), than that reported in detailed studies using TiO
2
2.2.2. Reactor
photocatalysis [14] or Fenton processes [15]. So far, there is no AOP known
that could be employed to eliminate 4 by oxidative degradation, but the
extent of its production could be dramatically diminished by working with
A 500 mL annular reactor (Pyrex, length: 30 cm, external diameter:
5 cm, Mangels, Bornheim, Germany) adapted for the immersion of the
Xe-excimer radiation source and equipped with an external electrode
[24] and a purging system was used. A built-in circulation system en-
sured efficient stirring of the solution. Experiments were performed by
irradiating 350 mL of solution within a temperature range of 25 to 30℃,
without adjusting the initial pH value of ca. 7.5.
VUV-radiation under conditions of O exclusion [11]. Up to the present time
2
no other AOP has been demonstrated capable to mineralize a s-triazine
moiety, they rather yield to the total conversion to cyanuric acid.
Aiming at a more detailed description of the reaction manifold of s-
triazines degradation induced by the VUV-photolysis of water, in compar-
ison to other AOPs, and with specific emphasis on the widely studied and
2
.2.3. Analyses
applied TiO -based photocatalysis, the present work deals with the VUV
2
HPLC analyses were carried out using a HP model 1090 liquid
photochemically initiated degradation of different s-triazine compounds,
exhibiting the same structure than atrazine but with different substituents
chromatograph with multiwavelength UV–vis detector (wavelength of
analysis: 220 nm); column: Lichrospher C18, length: 250 mm, i.d.:
(
melamine, 1, ammeline, 2, ammelide, 3 and cyanuric acid, 4, Scheme 1).
4
mm, 10 μm packing (Merck, Darmstadt, Germany,).
Melamine was chosen as the starting compound, because amino
substituents of the s-triazine moiety are also present in intermediates of
the oxidative degradation of atrazine, and compounds 2 and 3 were al-
ready found as intermediates of the oxidative degradation of atrazine and
of 1 [11,14–16]. Furthermore, when applying AOPs to melamine de-
gradation, cyanuric acid was obtained as the stable final product [15,16].
Moreover melamine based polymers find wide application, and even
if melamine is not classified as a health risk, its chronic toxicity is still
object of in-depth investigations [17].
2, 3 and 4 were analysed by Ion Interaction Chromatography using
the Immobilised Reagent Technique (IIIR). The column was loaded with
−2
a 1 × 10 M DDDMAB solution in acetonitrile/water (25%/75%) at
−
3
1
mL/min and washed with 30 mL of water; a 5 × 10 M phosphate
buffer solution at pH 7.7 was used as eluent at 1 mL/min. For 1, the
−
3
eluent was a solution of 1 × 10 M SDS and 0.1 M of NaCl in 15%
ethanol, adjusted to pH 3.06 with glacial acetic acid. Before the analysis,
the column was equilibrated for two hours with the eluent at 1 mL/min.
TOC (total organic carbon) analyses were carried out with a
Rosemount Analytical Instrument, Dohrmann DC-190.
Melamine has been implicated in diagnoses of urinary tract stones
and sand like calculi due to contamination of milk products [18].
Recently melamine degradation by means of TiO -based photo-
2
3
. Results and discussion
catalysis has been deeply investigated in order to give insights into the
mechanism of oxidation, and in particular into the role of hydroxyl ra-
dical vs direct hole oxidation mechanisms [16]. Indeed the role of bound
or free HO· mediated oxidation vs direct hole transfer oxidation has been
3
.1. VUV-photolysis versus UVC-photolysis of H
2
O
2
, photochemically
enhanced Fenton process and TiO
aspects
2
-photocatalysis: mechanistic and kinetic
largely debated in TiO -photocatalysis studies [19] and the elucidation of
2
the significance of these pathways has a fundamental importance in the
understanding and control of photocatalytic processes. In addition to
previously reported evidences in support of both mechanisms [20,21],
the photodegradation of melamine resulted as an efficient tool to eval-
uate the direct hole transfer ability of a photocatalyst, since it was de-
monstrated that melamine is not able to directly react with HO· radicals
Abiotic AOPs, and in particular those initiated by photochemical
means, relay on the generation of hydroxyl radicals, HO, to initiate
manifolds of oxidative degradation of organic compounds (pollutants)
dissolved in aqueous systems [2]. However, there are important dif-
ferences as far as the generation of reactive intermediates and their
(
local) concentrations are concerned.
Investigating on the UVC-photolysis of H
enhanced) Fenton process, H is added to the reaction system and is used
under H
2
O -UV irradiation or Fenton conditions.
2
2
O or on the (photochemically
2
The present work aims firstly to evaluate the possibility to attain
melamine mineralization by means of VUV-irradiation and secondly to
assess the degradation mechanisms in comparison with the findings ob-
2 2
O
%
to generate HO radicals. While for the first, HO radicals are generated by
the photochemical homolysis of H
2
O
2
(reaction 28), HO· radicals appear in
tained in TiO
2
-photocatalysis.
2+.
the second upon reduction of H O by Fe (reaction 29).
2
2
2