3.2 Degradation of 2,4-D photoinduced by
H2O2/goethite system in presence of
fluoride and bicarbonates at natural concentrations.
Fenton-like, photo-Fenton-like, and photocatalytic processes were
carried out with concentrations of goethite (α-FeOOH) often found in
natural well waters (0.3 mg L-1) in presence of
H2O2 (10 mg L-1), and
the effect of natural concentrations of fluoride (1.2 mg
L-1) and bicarbonate (86 mg L-1) was
evaluated (Figure 5).
2,4-D at concentrations of 30 mg L-1 did not undergo
degradation in water containing HCO3-(86 mg L-1) and F- (1.2 mg
L-1) by simulated sunlight irradiation (Figure 5:
HCO3-/F-/SL), such
as we have already reported under UV-B, UV-A, and visible light
irradiation, 2,4-D concentration was not strongly reduced in milli-Q
water suggesting that photolysis of this molecule did not play an
important role.
Although Lin and Gurol claimed that heterogeneous Fenton reaction could
take place in goethite/H2O2 systems
yielding hydroxyl (•OH) and peroxyl
(HO2•) radicals, the former able to
oxidize organic molecules (E°= 2.31 V vs. NHE) and the
latter with a lower oxidizing power , dark experiments showed here that
in presence of goethite (0.3 mg L-1/0.2 mg
L-1 of total iron),
H2O2 (10 mg L-1) and
anions (F- and
HCO3-), 2,4-D concentration was not
reduced (Figure 5:
G/F-/HCO3-/H2O2/Dark,
being G: goethite). In the aforementioned report, authors suggested
that, in dark conditions, H2O2 could be
adsorbed onto goethite surfaces and undergoes further decomposition
yielding •OH and
HO2• radicals (Eq. 1-3) (heterogeneous
Fenton). Low concentrations of goethite used herein would be responsible
for the minor role played by heterogeneous Fenton. Moreover, the dark
adsorption of 2,4-D on goethite (G/Dark) was found to be negligible.
\begin{equation}
H_{2}O_{2}\ \leftrightarrow(H_{2}O_{2})_{s}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)\nonumber \\
\end{equation}\begin{equation}
\equiv\text{Fe}^{3+}-OH+(H_{2}O_{2})_{s}\ \leftrightarrow\ \ \ \ \ \ \ \text{Fe}^{2+}+H_{2}O+HO_{2}^{\bullet}\text{\ \ }\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\nonumber \\
\end{equation}\begin{equation}
\equiv\text{Fe}^{2+}+\ H_{2}O_{2}\ \rightarrow\ \equiv\text{Fe}^{3+}-OH+^{\bullet}\text{OH}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\
\end{equation}
Photocatalytic reactions under sunlight irradiation (in absence of
H2O2) of goethite in the presence or
absence of anions led to a 2,4-D degradation of 10% after 240 min
(Figure 5: G/SL;
G/F-/HCO3-/SL). As
it was described above, α-FeOOH does not show an important
photocatalytic behavior since its conduction band potential is very
positive and unable to reduce molecular oxygen leading to the high
e-/h+ pair recombination despite
some authors have argued that goethite seems to be a promising
photocatalyst removing several organic pollutants such as dyes,
chlorophenols, and polycyclic aromatic hydrocarbons . However, in these
aforementioned studies, goethite concentrations ranged from 1 to 500 g
L-1 (herein it was used a concentration of 0.0003 g
L-1) and the high pollutant removal rates observed may
probably be directly related to high photocatalyst concentrations. In
addition, the photocatalytic reaction did not change in the presence of
bicarbonate and fluoride.
Otherwise, when 10 mg L-1 of
H2O2 was added into water containing
goethite (photo-Fenton-like/photocatalytic reactions) and bicarbonate at
initial pH 6.9 under simulated sunlight irradiation, 2,4-D concentration
was strongly reduced (75%) after 240 min of simulated sunlight
irradiation (Figure 5:
G/HCO3-/H2O2/SL).
Several authors have claimed the positive effect of
H2O2 addition into α-FeOOH
photocatalytic systems to degrade several pollutants . Nevertheless,
these studies were performed by using α-FeOOH and
H2O2 concentrations of 1 g
L-1 and 50-170 mg L-1 respectively
which are much higher than natural amounts of goethite (0.0003 g
L-1) and H2O2 (10 mg
L-1) used herein.
On the other hand, under sunlight irradiation, surface\(\equiv\text{Fe}^{2+}\)species could be photoinduced (Eq.4) . These
species could react with H2O2 leading to
the generation of surficial \(\equiv\text{Fe}^{3+}\)and extra•OH radicals (heterogeneous photo-Fenton) (Eq. 4).
\begin{equation}
\equiv\text{Fe}^{3+}-OH+hv\ \rightarrow>\equiv\text{Fe}^{2+}+\ {\text{OH\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(4\right)}\ \nonumber \\
\end{equation}
Furthermore, surficial \(\equiv\text{Fe}^{3+}-OOH\) species
generated by H2O2 adsorption on goethite
surface could (Eq. 5), by UV-vis light irradiation, to photo-induce
ferryl species (\(\equiv\text{Fe}^{4+}=O)\), this latter very
unstable in aqueous solutions, also leading to the formation of•OH radicals (Eq. 7) .
\begin{equation}
\equiv\text{Fe}^{3+}-OH+\ H_{2}O_{2}\rightarrow\ \equiv\text{Fe}^{3+}-OOH+\ H_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)\nonumber \\
\end{equation}\begin{equation}
\equiv\text{Fe}^{3+}-OOH+hv\ \rightarrow\equiv\text{Fe}^{4+}=O+^{\bullet}\text{OH}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (6)\nonumber \\
\end{equation}\begin{equation}
{\ \equiv Fe}^{4+}=O+\ H_{2}O\ \rightarrow\ \equiv\text{Fe}^{3+}-OH+\ ^{\bullet}\text{OH}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (7)\ \nonumber \\
\end{equation}
Bicarbonates (83.6 mg L-1) which were also present at
typical concentrations often found in natural waters regulating the pH
at 6.9, could react with •OH radicals generating
carbonate radicals (CO3-•) (Eq. 8)
able to remove 2,4-D as well . CO3-•radicals could participate in electron transfer and H-abstraction
reactions leading to the oxidation of organic molecules . All these
routes mentioned above could be responsible for the significant removal
of 2,4-D in presence of bicarbonate.
\begin{equation}
\text{HCO}_{3}^{-}+\ ^{\bullet}\text{OH}\ \rightarrow\ \text{CO}_{3}^{-\bullet}+\ H_{2}O\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (8)\nonumber \\
\end{equation}
Surprisingly, experiments, where only hydrogen peroxide, fluoride, and
bicarbonate were present (in absence of goethite) at concentrations of
10, 83.6, and 1.2 mg L-1 respectively under simulated
sunlight irradiation at initial pH 6.9, exhibited a strong 2,4-D removal
of around 85% after 240 min (Figure 5:
H2O2/F-/HCO3-/SL).
As it has been suggested, sunlight or UV-B light irradiation can
photo-induce H2O2 photolysis (Eq. 9) .
Pyrex-glass reactors used can allow the transmission to a lesser extent
of UV-B light coming from the solar simulator leading to the
H2O2 photolysis.
\begin{equation}
H_{2}O_{2}+hv\ \left(280-315\ nm\right)\ \rightarrow 2^{\bullet}\text{OH}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (9)\nonumber \\
\end{equation}
The experiments of H2O2 photolysis
without anions (H2O2/SL) revealed that
2,4-D degradation underwent a slight reduction. Perhaps, bicarbonate
reaction with •OH radicals leading to the generation
of CO3-• could be behind the observed
enhancement (Eq. 8).
The photocatalytic experiment in absence of fluoride
(G/HCO3-/H2O2/SL)
showed evidence about goethite could affect the •OH
radical production by H2O2 photolysis.
Two phenomena could be responsible for this detrimental effect: (i)
H2O2 oxidation (+1.06 V vs NHE) by
photoinduced goethite valence band holes (VB oxidation potential: +2.3 V
vs NHE ) could also take place producing a low oxidant peroxyl radical
(HO2•) (Eq. 10; Figure 7) unable to
oxidize 2,4-D. Peroxyl radical is in equilibrium with superoxide radical
(O2-•) (Eq. 11) and its
pKa is around 4.8, so at pH higher than
pKa (pH during experiments increased from 6.9 to 7.5)
superoxide radical will be the main specie in solution which undergoes a
fast disproportion leading to the generation of
H2O2 (Eq. 12-13) [43].
\begin{equation}
{H_{2}O_{2}+\ h_{\text{VB}}^{+}\ \rightarrow\ \text{HO}_{2}^{\bullet}+H^{+}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (10)\backslash n}{\text{HO}_{2}^{\bullet}\ \leftrightarrow\ H^{+}+\ O_{2}^{-\bullet}\text{\ \ \ \ \ \ \ \ \ \ \ \ p}K_{a}=4.8\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (11)}\nonumber \\
\end{equation}\begin{equation}
\text{HO}_{2}^{\bullet}+\text{HO}_{2}^{\bullet}\ \rightarrow\ H_{2}O_{2}+\ O_{2}\ k=8.6x10^{5}M^{-1}s^{-1}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (12)\ \ \ \ \ \ \ \ \ \ \nonumber \\
\end{equation}\begin{equation}
O_{2}^{-\bullet}+\ O_{2}^{-\bullet}+\ {2H}^{+}\rightarrow\ \ H_{2}O_{2}+\ O_{2}\ k=\ll 100\ M^{-1}s^{-1\ }\ \ \ \ \ \ \ \ \ (13)\ \ \ \nonumber \\
\end{equation}
On the other hand, (ii) goethite exhibits an important UV light
absorption (Figure 3b) generating possibly a screen effect and competing
with H2O2 molecules by UV-B photons.
Another pathway for ROS generation could be
H2O2 direct reduction by goethite CB
photoinduced electrons. Since hydrogen peroxide reduction potential is
+0.32 V (vs NHE) and photoinduced conduction band electrons in goethite
may have a redox potential of +0.24 V (vs NHE) [44], it is expected
that H2O2 reduction by goethite CB
photoinduced electrons takes place leading to the generation of hydroxyl
radicals (eq. 14).
\begin{equation}
H_{2}O_{2}{\ +\ e}_{\text{CB}}^{-}\rightarrow\ ^{\bullet}{OH+\ \text{OH}^{-}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(14\right)}\nonumber \\
\end{equation}
As was expected, when natural amounts of fluoride (1.2 mg
L-1) were added to water containing already
bicarbonates (83.6 mg L-1) in presence of goethite and
H2O2 (10 mg L-1), the
2,4-D degradation increased markedly reaching 95% in 240 min of
simulated sunlight irradiation (Figure 5:
G/F-/HCO3-/H2O2/SL).
XPS measurements of goethite separated from the aqueous solution after
the reaction confirmed the presence of fluoride on goethite (1.2 %At)
(Figure 6a) and a high-resolution F 1s spectrum (Figure 6b) revealed the
presence of two signals at 684.7 eV and 689.5 eV often linked to the
presence of adsorbed fluoride in iron (hydr)oxides and Fe-F bonds
respectively .
Hiemstra and Van Reimsdijk concluded that fluoride adsorption onto
goethite is achieved on several surface sites being benefited at acidic
pH values (goethite isoelectric point 7.9). Moreover, the authors
suggested by theoretical calculations that the formation of surface Fe-F
bonds would be feasible. On the other hand, Ding et al. corroborated
that the highest fluoride adsorption was obtained at acid pH values and
proposed that fluoride adsorption may be achieved by exchange with
surficial -OH groups. In other ways.
As it was above mentioned, the beneficial effect of fluoride presence
over photocatalytic degradation using goethite and lepidocrocite was
first claimed by Du et al. . The use of 8.5-85 mg L-1fluoride increased the photocatalytic degradation of orange II dye at pH
6.5 by irradiation of these iron (hydr)oxides (0.5 g
L-1). Authors suggested that fluoride could modify the
surface of iron (hydr)oxides increasing the production of•OH radicals, however, the mechanism was not proposed.
Meanwhile, on TiO2 a mechanism was suggested. In the
literature there are several studies suggesting different mechanisms
about how fluoride anions could enhance the photocatalytic activity of
metal oxide semiconductors as TiO2. For instance,
Minella et al., suggested that the flat band potential of semiconductors
may be fine-tuned by surficial anion adsorption. This flat band
modification will depend on the nature and density of ions. Otherwise,
Minero et al. , indicated that fluoridation of TiO2could enhance the photocatalytic activity of the metallic oxide towards
the phenol degradation due to the formation of Ti-F surface sites.
Authors suggested that the exchange of surficial -OH groups by F atoms
on TiO2 may decrease the formation of deeply trapped
holes, favoring less deep surface trapping sites and making faster the
electron transfer of photoinduced holes to phenol molecules.
In addition, a systematic study reported by Deskins and coworkers where
DFT calculations were carried out, demonstrated that rutile
TiO2 surfaces could be strongly modified by adsorbates
(especially those molecules highly electronegative) tailoring the
surface chemistry. Later, Montoya and Salvador measured the flat band
potential of fluorine-modified TiO2 finding that this
underwent a negative shifting of about 80 mV compatible with upward band
bending in semiconductors by an excess of negative charge in the
surface. For their part, Xu et al. proposed a new theory where they
suggested that fluoride ions present in the Helmholtz layer can induce
the desorption of surface-bound •OH radicals
photoinduced in TiO2 surfaces through a fluorine
hydrogen bond (Figure 6). This latter could support our results where
the presence of two different species of fluoride interacting with the
goethite surface were found by XPS.
Other authors have also suggested that the presence of surficial\(\equiv Ti-F\) species may play an important role since photoinduced
VB holes may be unable to oxidize the F-(E○(F•/F-)=3.6 V
vs NHE) and would react directly with water molecules yielding free•OH radicals .
\begin{equation}
\equiv Ti-F+\ h_{\text{VB}}^{+}\rightarrow\ \equiv Ti-F+\ ^{\bullet}\text{OH}+\ H^{+}\ \ \ \ \ \ \ \ \ \ (15)\nonumber \\
\end{equation}
Thus, it is possible suggesting that α-FeOOH surfaces in presence of
fluoride (at natural concentrations) can be superficially modified
leading to either/both upward band bending or/and enhanced generation of
free hydroxyl radicals upon sunlight irradiation (Figure 7). Upward band
bending could enhance the H2O2 reduction
by photo-induced conduction band electrons in goethite (Eq. 14) yielding
more efficiently •OH radicals, therefore, enhanced
photocatalytic degradation of 2,4-D, such as was observed.
The effect of homogeneous photo-Fenton reactions was also assessed since
often iron (hydr)oxides can undergo photoinduced iron dissolution in
water . Factors such as acidic pH and the presence of aliphatic acids as
oxalate (by forming soluble ferric-oxalate complexes at circumneutral
pH, for this reason, herein photocatalytic experiments were not carried
out in absence of bicarbonates, since under these conditions, the pH of
the solution dropped reaching values about of 4.0 promoting goethite
dissolution) can also enhance iron dissolution. The presence of
dissolved iron either complexed or not can induce homogeneous
photo-Fenton reactions in presence of hydrogen peroxide . In our
experimental conditions, the final pH of the reaction was always around
7.5 confirming that 2,4-D removal was achieved at neutral/basic pH. The
presence of dissolved iron was evaluated by atomic absorption
spectroscopy measurements of the supernatant obtained by further 0.22 µm
filtration after the photochemical and Fenton-like reactions. In dark
experiments, the total iron concentration was 2.81 µg
L-1 while the simulated sunlight-irradiated experiment
in absence of hydrogen peroxide showed an iron concentration of 2.9 µg
L-1. The simultaneous presence of goethite, anions
(F- and HCO3-),
H2O2, and simulated sunlight exhibited
an iron concentration of 11 µg L-1. These iron amounts
are too low to induce Fenton or photo-Fenton reactions at neutral pH
able to generate appreciable concentrations of •OH
radicals responsible for herbicide removal; pointing out that the
degradation was mainly due to heterogeneous photo-Fenton
processes/sunlight H2O2 photolysis.