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H. Zúñiga-Benítez, G.A. Peñuela / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 62–70
of water/0.01% formic acid and methanol/0.01% formic acid. The
organic phase fraction was maintained at 50% (isocratic mode).
Ethylparaben-d4 (CDN isotopes) was used as internal standard.
Analysis run time was 2.5 min, and the injection volume was 5 mL.
Degradation by-products were analyzed by GC–MS in an
Agilent 7890A gas chromatograph coupled to an Agilent 5975C
mass spectrometer using a programmed temperature-vaporizing
injector (PTV). The column employed for chromatographic
separation was an HP-5MS UI (30 m, 0.25 mm, 0.25 mm, Agilent
Technologies).
positively (pH < 6.5) or negative (pH between 6.5 and 7.5), which
means that there is not a significant electrostatic interaction
between these species. In this regard, results associated with the
pH range 3–7.5 could be related to the fact that at higher pH values,
there is a higher concentration of hydroxide anions (OHꢁꢁ), which
promotes the generation of hydroxyl radicals (Eq. (3)) that
eventually contributes to the contaminant oxidation [20]. In the
opposite case, at a higher pH most of the paraben has a negative
polarity, promoting a repulsive interaction Eth-PB-TiO2 surface,
which would probably be responsible for the decrease in the extent
of substrate removal. Additionally, in an alkaline medium, H2O2 is
highly unstable, conducting to its self-decomposition, which
implies a reduction of a potential additional source of ꢂOH and,
hence the pollutant removal rate [15].
2.5. Mineralization and biodegradability studies
Organic matter mineralization was evaluated analyzing the
changes in the dissolved organic carbon (DOC) present in the
solutions, using an Apollo 9000 series TOC analyzer (Teledyne
Tekmar). Biodegradability index (biochemical oxygen demand
(BOD5)/chemical oxygen demand (COD) ratio) was determined
according to the Standard Methods for the Examination of Water
and Wastewater (2012) [18], methods 5220 D and 5210 D.
3.1.2. Effect of TiO2 initial concentration
According to Fig. 2a, the catalyst concentration plays an
important role on pollutant photo-treatment. Initially, increasing
this parameter from 0.2 to ꢀ1.0 g Lꢁ1 leads to an increase in the
extent of contaminant degradation (from ꢀ62% to ꢀ77% respec-
tively). This situation is associated with the fact that when the
concentration of the semiconductor increases, more photons may
be absorbed, representing a higher number of active sites and a
possible increment in the generation of ꢂOH radicals and other
reactive species that eventually could contribute to the paraben
removal [20,21]. However, at higher catalyst loads, aggregation of
TiO2 particles would imply a possible reduction and scattering of
the light that penetrates to the solution, which is reflected in a
decrease in the Eth-PB removal as the figure indicates [20–22].
3. Results and discussion
3.1. Ethylparaben photocatalytic removal at lab-scale
3.1.1. Effect of solution initial pH
The effects of solution pH, catalyst and H2O2 initial concen-
trations on pollutant elimination were assessed considering the
tests and levels reported in Table 1. In the same way, Fig. 2a
represents the main effects plot for Eth-PB removal. From the
figure, it can be noted that changes in the initial pH of the solution
influence markedly the extent of pollutant removal after 30 min of
photo-treatment. Initially, at pH values between 3 and ꢀ7.5, an
increment of this parameter implies a higher Eth-PB degradation.
But, if the pH is higher, an inhibitory effect can be appreciated. This
situation could be associated with the different electrostatic
interactions between the pollutant and the catalyst surface that
can occur during the reaction. TiO2 Degussa P-25 has a point of zero
charge (pzc) in the range 5.7–6.5 [19]. It means that at pH < pzc the
catalyst surface is positively polarized, while at pH > pzc the TiO2
charge is negative. For its part, the acid dissociation constant (pKa)
of ethylparaben is 8.22 [5], implying that at pH conditions greater
than this; the pollutant will be found primarily in its anionic form.
Thus, at pH lower than 7.5, it is expected that almost all the Eth-PB
is in its molecular form, while the catalyst surface is charged
3.1.3. Effect of H2O2 initial concentration
Similar to the results related to the influence of TiO2 on
substrate removal, Fig. 2a shows that variations in the initial
concentration of H2O2 in the solution are associated with both
positive and negative effects in terms of Eth-PB elimination. In the
first case (H2O2 concentration between 50 and ꢀ125 mg Lꢁ1),
presence of H2O2, could reduce the possibility of an electron–hole
ꢂ
pair recombination, which would promote the generation of OH
radicals as has been reported previously. H2O2 also can react with
other species present in the solution to form additional reactive
agents that would contribute to the contaminant degradation [22].
On the other hand, the excess of H2ꢂO2 (in this case concen-
trations >125 mg Lꢁ1) may scavenge the OH free radicals present
in the solution, reducing significantly the pollutant oxidation
[15,22].
Table 1
Experimental design for pollutant removal using TiO2/H2O2 heterogeneous photocatalysis (pollutant initial concentration: 1.0 mg Lꢁ1, temperature: 35 ꢃ 2 ꢄC, irradiance:
350 W mꢁ2, irradiation time: 30 min).
Experiment
pH
TiO2 concentration (g Lꢁ1
)
H2O2 initial concentration (mg Lꢁ1
)
Eth-PB removal (%)
1
2
3
4
5
6
7
8
9
3
9
6
6
3
6
9
3
6
9
6
3
3
6
6
9
1.5
1.5
0.85
0.85
0.2
1.5
0.85
1.5
0.2
1.5
0.2
0.85
0.2
0.85
0.85
0.85
0.2
50
65.9
68.8
80.4
75.6
67.5
60.7
78.4
69.2
59.8
65.2
63.6
71.5
35.6
63.4
76.8
72.4
55.2
150
100
100
100
50
100
150
150
100
150
50
9
10
11
12
13
14
15
16
17
50
100
100
150
50
Zú?iga-Benítez, Henry
