GModel
CATTOD-8604; No. of Pages7
ARTICLE IN PRESS
2
C. Zhao et al. / Catalysis Today xxx (2013) xxx–xxx
OH
C
OH
OH
H C
H
H
3
H C
H
2
O
OHC
O
O
O
3
SO
O
2
SeO2
CH COOH
NaBH
4
O
HN
NH
MeOH
HN
NH
HN
NH
3
N
NH
HN
NH
HN
NH
H C
O
3
O
O
H
NH
O
O
Fig. 2. Synthesis of 6-hydroxymethyl uracil.
Fig. 1. Structure of cylindrospermopsin (CYN) and the model compound 6-
hydroxymethyl uracil (6-HOMU).
and S-TiO2 under 350, 420 and 450 nm irradiation. Among these
photocatalysts, NF-TiO2 was the most UV and visible light active
for the degradation of the CYN model compound. The treatment is
effective over a range of pH, and in the presence of ions and HA.
Advanced oxidation processes (AOPs) have received consider-
able attention for the treatment of water contaminated with a wide
variety of toxins and pollutants. AOPs employ the hydroxyl radical
•
−
O2
appears to play the predominant role in the VLA photocat-
•
alytic degradation process. Our results demonstrate NF-TiO2 can
be used for the UV and visible light photocatalytic destruction of
uracil based compounds under a variety of conditions.
(
HO ) as the primary oxidant in the oxidative remediation of toxins
•
in drinking water. HO is a very powerful oxidant, which can react
rapidly with nearly all organic compounds due to its strong oxi-
dation potential (+2.80 V) [14]. UV TiO2 photocatalysis is effective
for the destruction of an extensive number of toxins and organic
contaminants in the wastewater and drinking water [15,16]. Con-
2. Materials and methods
2.1. Materials
ventional UV TiO photocatalysis can produce a number of reactive
2
•
oxygen species (ROS), however HO is generally associated with
FeCl ·6H O, CuCl ·2H O, CaCl ·2H O, MgCl as the sources
the effective destruction of organic compounds. UV photoexcita-
3
2
2
2
2
2
2
−
+
of common ions, acetic acid, sodium bisulfite and HPLC grade
methanol were purchased from Fisher Scientific. The humic acid
was obtained from Fluka. 6-Methyl uracil and selenium dioxide
were purchased from Sigma Aldrich. Sodium borohydride was
obtained from Acros organics. All reagents were used as received.
The synthesis and characterization of the doped-TiO2 materials
tion of TiO2 produces electron/hole (ecb /hvb ) pairs as illustrated
−
by Eq. (1). The ecb can reduce molecular oxygen yielding super-
oxide anion radical (O2 ), Eq. (2) and the hvb+ has the potential to
•−
•
oxidize surface absorbed H O or hydroxyl groups to generate HO ,
Eq. (3). Another source of HO can occurs via disproportionation of
O2 , yielding H O , Eq. (4), which can be reduced to HO Eq. (5).
2
•
•
−
•
2
2
(
NF-TiO , PF-TiO , and S-TiO ) are reported elsewhere [17,18].
2 2 2
−
+
TiO + h → e
+ hvb
(1)
(2)
(3)
(4)
(5)
2
cb
•−
The model compound (6-hydroxymethyl uracil) was synthesized
according to standard organic functional group transformations.
All aqueous solutions were prepared with Millipore filtered water.
−
ecb + O → O
2
2
hvb + H O → H + HO•
+
+
2.2. Sample preparation
2
•
−
−
The loading of doped-TiO2 materials employed for UV
photocatalysis experiments was 0.05 g/L with the initial model
compound concentration of 5 ppm unless otherwise stated. VLA
2
O2 + 2H O → H O + 2OH + O2
2
2
2
−
cb
→ OH− + HO•
H O + e
2
2
TiO photocatalysis of model compound was conducted with 0.1 g/L
2
Limitations effecting the broad application of UV TiO2 photo-
catalysis include the requirement of costly UV light (<387 nm) and
NF-TiO2 and 1 ppm initial concentration of model compound.
tor (12 × 1 in., 150 mL capacity, with a vented Teflon screw top)
and magnetically stirred in the dark and purged with oxygen for
15 min prior to irradiation and during irradiation. A Rayonet pho-
tochemical reactor (Southern New England Ultra Violet Company,
www.rayonet.org, model RPR-100) was used for all experiments,
equipped with a cooling fan and 15 phosphor-coated inter change-
able lamps at ꢀ = 350, 420 and 450 nm. Aliquots (1 mL) were
collected at given time intervals and filtered through a 0.45 m
filter prior to high-performance liquid chromatography (HPLC)
analysis.
−
+
the rapid recombination of ecb /hvb pairs, leading to low quantum
yields. The wavelengths of solar irradiation that reach the surface
of the earth are mostly in the visible region (40%) with a small frac-
tion (5%) in UV region. Hence, photocatalysts activated by visible
light or by a broad spectrum of wavelengths (solar) have signifi-
cant economic advantages. Doping of TiO2 (NF-TiO , PF-TiO2 and
2
S-TiO ) can result in a decrease band gap such that longer wave-
2
length light (visible and solar light) becomes applicable [17,18].
−
+
In addition, doped TiO2 can inhibit ecb /hvb pairs recombination
through trapping of charge carriers. VLA photocatalysis employ-
ing TiO based materials has received significant attention recently
2
and also been reviewed by the research groups of Zhao [19] and
Dionysiou [20]. UV and VLA TiO2 photocatalysis are effective for
treatment of microcystin cyanotoxins (MC), however only a limited
number of reports have appeared on the photocatalysis of CYN [21].
The high cost of CYN limited our ability to conduct detailed stud-
ies to optimize the reaction conditions for the TiO2 photocatalytic
destruction of CYN. The uracil moiety in CYN is critical to the toxic-
2.3. Preparation of model compound
The synthesis of 6-HOMU includes two steps (Fig. 2): synthe-
sis of orotaldehyde and reduction of orotaldehyde to 6-HOMU. The
orotaldehyde was prepared by Kwang-Yuen’s method [23]. Briefly,
6-methyl uracil (2.54 g) was refluxed in acetic acid (60 mL) with
selenium dioxide (2.66 g) for 6 h. The hot reaction mixture was fil-
tered and the yellow filtrate collected and solvent evaporated. The
crude orotaldehyde was then dissolved in hot water (24 mL) and
gravity filtered to remove the carbon. The filtrate was acidified to
•
ity of CYN. HO reacts primarily (84%) at the uracil ring of CYN [22].
With this in mind, 6-hydroxymethyl uracil (6-HOMU) (Fig. 1) was
synthesized and used as a model compound for the UV and VLA
photocatalysis of CYN. We report herein the photocatalytic activ-
ity of different non-metal doped TiO2 materials, NF-TiO , PF-TiO2
2
Please cite this article in press as: C. Zhao, et al., UV and visible light activated TiO2 photocatalysis of 6-hydroxymethyl uracil, a model compound