Degradation of organochlorinated pollutants in water by catalytic hydrodechlorination and photocatalysis

Article abstract of DOI:10.1016/j.cattod.2015.08.013

The degradation of chlorinated herbicides (MCPA and 2,4-D) and 4-chlorophenol (4-CP) by photocatalytic oxidation (PCO) and the combination of catalytic hydrodechlorination (HDC) and photocatalysis, at ambient conditions, has been studied. Commercial TiO₂ (P25) and Pd/Al₂O₃ catalysts were used for PCO and HDC, respectively. MCPA and 2,4-D were transformed upon photo-oxidation to intermediate products and almost total mineralization was achieved. However, in the case of 4-CP, a conversion of only 82% of chloride formation and 87% TOC were obtained. In spite of the fact that the HDC reaction resulted in a total dechlorination of organochlorinated pollutants combined with an important decrease of the effluent ecotoxicity, the percentage of mineralization obtained in the combined process (HDC-PCO) was slightly lower than in the PCO treatment. Thus, the HDC-PCO process is not justified versus a single PCO treatment.

Full text of DOI:10.1016/j.cattod.2015.08.013

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Contents lists available at ScienceDirect
Catalysis Today
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Degradation of organochlorinated pollutants in water by catalytic
hydrodechlorination and photocatalysis
Elena Diaza, Marina Cebriana, Ana Bahamondeb, Marisol Faraldosb,
Angel F. Mohedanoa,∗, Jose A. Casasa, Juan J. Rodrigueza
a Sección de Ingeniería Química, Facultad de Ciencias, Universidad Autónoma de Madrid, C/ Francisco Tomás y Valiente 7, 28049 Madrid, Spain
b Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2015
Received in revised form 25 July 2015
Accepted 3 August 2015
Available online xxx
The degradation of chlorinated herbicides (MCPA and 2,4-D) and 4-chlorophenol (4-CP) by photocat-
alytic oxidation (PCO) and the combination of catalytic hydrodechlorination (HDC) and photocatalysis, at
ambient conditions, has been studied. Commercial TiO2 (P25) and Pd/Al2O3 catalysts were used for PCO
and HDC, respectively. MCPA and 2,4-D were transformed upon photo-oxidation to intermediate prod-
ucts and almost total mineralization was achieved. However, in the case of 4-CP, a conversion of only
82% of chloride formation and 87% TOC were obtained. In spite of the fact that the HDC reaction resulted
in a total dechlorination of organochlorinated pollutants combined with an important decrease of the
effluent ecotoxicity, the percentage of mineralization obtained in the combined process (HDC–PCO) was
slightly lower than in the PCO treatment. Thus, the HDC–PCO process is not justified versus a single PCO
Keywords:
MCPA
2,4-D
4-Chlorophenol
Herbicides
treatment.
© 2015 Elsevier B.V. All rights reserved.
Hydrodechlorination
Photocatalysis
1. Introduction
Many research efforts have been devoted to developing pro-
cesses for the removal of these compounds from soil and water
Nowadays the extensive and sometimes excessive use of pes-
ticides has led to surface water and groundwater pollution.
Chlorophenoxy herbicides, like 4-chloro-2-methylphenoxyacetic
acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D), are com-
monly used for the control of broadleaved weeds in a variety of
places including home lawns, cereal and grain crops, commer-
cial areas, commercial turf, and forests [1]. Consequently, it may
be washed down into surface waters, mainly in the anionic form.
In addition, chlorophenols which occur as end products or inter-
mediates in the manufacture of herbicides, disinfectants, wood
preservatives, personal care formulations, dyes and wood preser-
vatives can be also formed by the chlorination (during disinfection
of water and wastewater) of humic matter [2]. These compounds
are usually found in aqueous wastes from cleaning herbicide con-
tainers in the agricultural industries and in wastewaters from
herbicide manufacturing plants in a wide range of concentrations
(1–1000 mg L−1) and at low concentrations in municipal wastewa-
ter treatment plants [3,4].
by means of biological, chemical and photochemical methods [5].
In this context, catalytic hydrodechlorination (HDC) represents a
detoxifying technology for the conversion of organochlorinated
pollutants to less toxic compounds (non-chlorinated compounds)
[6,7]. This work has demonstrated its ability to deal with aliphatic
and aromatic organochlorinated compounds, such as chloroeth-
lylenes, chlorobenzenes or chlorophenols or even more complex
molecules such as the chlorinated herbicides alachlor, diuron or
clopyralid in water using supported catalysts based on precious
metals, especially Pd, as the active phase [8–11].
Meanwhile, Advanced Oxidation Processes (AOPs) are effective
remediation methods which use high oxidation-potential sources
to produce the primary oxidant species, the hydroxyl radical (•OH),
which reacts rapidly, and unselectively, with most organic com-
pounds. As a result of the non-specific and high electron affinity of
the hydroxyl radical, the degradation products can be hydroxylated
or partially oxidized intermediates, dimerized compounds, carbon
dioxide, and mineral acids [12].
In the case of photocatalytic oxidation (PCO), the photoexcita-
tion of a semiconductor (mostly TiO2) under irradiation with light of
suitable wavelength, generates an e−/h+ pair, creating the potential
for both reduction and oxidation processes to occur at the surface
of the semiconductor, for nearly all substrates investigated [13].
∗
Corresponding author.
E-mail address: angelf.mohedano@uam.es (A.F. Mohedano).
http://dx.doi.org/10.1016/j.cattod.2015.08.013
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Diaz, et al., Degradation of organochlorinated pollutants in water by catalytic hydrodechlorination
and photocatalysis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.013

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Although a number of possible degradation pathways can be envi-
sioned, the formation and subsequent reactions of the hydroxyl
radicals, very strong oxidizing agents, generated from the oxida-
tion of water molecules and hydroxyl ions by photo-excited TiO2,
are generally accepted as the predominant degradation pathway
of organic substrates in oxygenated aqueous solutions [14–16]. In
particular, halogenated organic compounds (alkanes, alkenes and
aromatic compounds) were reported to undergo complete miner-
alization in water suspensions of TiO2, with the formation of CO2,
H2O and mineral acids [17].
In spite of the fact that it has been shown that some organochlo-
rinated pollutants, such as 4-chlorophenol (4-CP) or 2,4-D, can
be successfully degraded by various AOPs, including the Fenton
reaction [18], photo-Fenton [5], UV/H2O2 process [19,20], het-
erogeneous photocatalysis (UV/TiO2) [21,22], anodic Fenton [23],
ozonation [24], and catalytic ozonation [25], the oxidation of these
pollutants requires special attention because in some cases, the tox-
icity of the original effluents can increase by the formation of toxic
intermediates [26,27].
During the last decade, there has been a growing interest in the
development of more efficient strategies to treat wastewater with
organochlorinated compounds. The use of sequential or simulta-
neous hybrid configurations can improve both their detoxification
and final mineralization [28,29]. The combination of an AOP process
with a biological treatment to achieve complete mineralization at
moderate costs is really interesting but it is necessary that the AOP
treatment involved provides a highly biodegradable effluent with-
out components which are toxic to microorganisms [26]. On the
other hand, a hybrid AOP process based on the combination of cat-
alytic wet peroxide oxidation (CWPO) and photocatalysis produced
a rapid breakdown of the aromatic compounds, associated with
the CWPO process, and effective mineralization of the resulting
low molecular weight carboxylic acids (LMWCA) by photocatalytic
oxidation [30]. Another strategy is to couple an advanced reduc-
tive catalytic process, such as hydrodechlorination, with an AOP.
In this case, the combination of HDC followed by CWPO has been
found to be an effective solution for the abatement of chlorophe-
nols in water [27]. HDC undertakes an essential detoxification by
means of the transformation organochlorinated compounds into
their dechlorinated species and avoids the formation of conden-
sation by-products, and CWPO leads to high mineralization in a
shorter time than by CWPO alone.
To minimize light scattering by the photocatalyst particles and to
avoid appreciable saturation by the substrate, 200 mg L−1 of TiO2
P25 was employed. In all cases the inlet chlorinated herbicide con-
centrations had 50 mg L−1 Total Organic Carbon (TOC), correspond-
ing to 115, 93 and 89 mg L−1 2,4-D, MCPA and 4-CP, respectively.
The HDC runs were performed in a semicontinuous stirred
tank reactor from Autoclave Engineers using a Pd/Al2O3 catalyst
with a metal load of 0.5% (w/w) supplied by BASF (BET surface
area ≈ 92 m2 g−1; pore volume ≈ 0.36 cm3 g−1). The Pd-␥-alumina
particles were provided as 2.4–4 mm diameter egg-shell spheres
which were pulverized and a powdered Pd/Al2O3 supported cat-
alyst was always used (dp < 100 ␮m). An aqueous solution of the
original organochlorinated compounds 50 mg L−1 TOC, was placed
in the reactor and hydrogen was continuously fed at a flow rate
of 25 N mL min−1. A temperature of 30 ◦C, a pressure of 1.2 bar,
200 mg L−1 of catalyst loading and a stirring velocity of 700 rpm
were always used. A constant gas flow rate and reactor pressure
were maintained by means of a mass flow controller and a back-
pressure control valve, respectively. The reactor was heated to the
reaction temperature, which was measured and controlled by a
thermocouple in the liquid phase.
Liquid samples were periodically taken from each reactor, the
catalyst was separated by filtration using a 0.2 ␮m pore size PTFE
filter and analyzed. The reaction compounds were analyzed by GC
with a flame ionization detector (GC 3900 Varian) using a 30 m
length × 0.25 mm i.d. capillary column (CP-Wax 52 CB) and by HPLC
(Varian Prostar 325) with a UV detector using a C18 as stationary
phase (Valco Microsorb-MW 100-5 C18) at 280 nm and a mix-
ture of acetonitrile:acidic water (acetic acid 0.1 wt.%) as the mobile
phase at 0.5 mL min−1. LMWCA and chlorides were analyzed by an
Ion Chromatograph with chemical suppression (Metrohm 883 IC)
and a conductivity detector using a Metrosep A supp 7-250 col-
umn (250 mm length, 4 mm diameter) as the stationary phase. The
TOC content of the aqueous samples was also quantified using an
infrared-detector TOC-VCSH/CSN Shimadzu analyzer. The pH was
measured with a pH meter (CRISON). Ecotoxicity measurements
were carried out using a bioassay following the standard Microtox
test procedure (ISO 11348-3, 1998) [31], based on the decrease in
light emission by the marine bacteria Vibrio fischeri (Photobacterium
phosphoreum), using a Microtox M500 Analyzer (Azur Environmen-
tal).
All the experiments were performed in duplicate and the data
reproducibility was always better than 5%.
The aim of this work is to evaluate and compare the performance
of the photocatalytic oxidation of three chlorinated pollutants,
2,4-D, MCPA and 4-CP, with a sequential process based on a first
step of catalytic hydrodechlorination, with a Pd/Al2O3 catalyst, fol-
lowed by heterogeneous photocatalysis, withTiO2 P25. The major
drawbacks and benefits presented by both a single and a coupled
advanced catalytic processes (HDC and PCO) have been analyzed.
3. Results and discussion
3.1. Single photocatalysis process
Fig. 1 shows the results of the photocatalytic degradation for
2,4-D, MCPA and 4-CP, expressed as TOC concentration. The reduc-
tion in TOC during irradiation time confirmed that 2,4-D (Fig. 1A),
and MCPA (Fig. 1B), were being transformed by photo-oxidation to
intermediate products which evolve to CO2 and H2O as the reac-
tion proceeds, while the results of 4-CP photodegradation (Fig. 1C)
presented a parallel evolution for TOC and 4-CP concentration
throughout the reaction, which indicates the presence of minor
photo-oxidized by-products.
With regard to the presence of photo-oxidized intermediates
that could contribute to the modification of toxicity during the
photodegradation reaction [32], 2,4-D concentration dramatically
decreased at 60 min when 2,4-dichlorophenol was found to be the
major aromatic intermediate, which is more persistent and toxic
than 2,4-D [33]. Furthermore, 2,4-dichlorophenol (whose forma-
tion can be explained by considering the attack of an •OH radical
on the alkyl chain of the molecule) and other minor intermediate
2. Material and methods
Two different processes were carried out: (a) the single process,
based on a photocatalytic only run, and (b) the sequential process,
consisting of a first HDC stage followed by a second stage of PCO
run which is fed with the effluent leaving the HDC treatment.
PCO experiments were carried out with a commercial tita-
nia catalyst Evonik TiO2 P25 (BET surface area ≈ 55 m2 g−1; pore
volume ≈ 0.65 cm3 g−1) which presents a mix of crystalline struc-
ture, 85% of anatase phase and 15% of rutile. The processes
were performed in a semicontinuous slurry-photoreactor set in
a Multirays apparatus (Helios Italquartz) enclosed by ten 15 W
fluorescent lamps (6 UV blacklight lamps and 4 Day-light lamps)
with 38.4 W m−2 irradiance measured by a Kipp & Zonen model
CUV-4 broadband UV radiometer with UV range (306–383 nm).
Please cite this article in press as: E. Diaz, et al., Degradation of organochlorinated pollutants in water by catalytic hydrodechlorination
and photocatalysis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.013

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Table 1
3
A
TOC
TOC initial photodegradation rates for 2,4-D, MCPA and 4-CP using PCO and the
combination of HDC and PCO.
TOC 2,4-D
TOC 2,4-dichlorophenol
TOC LMWCA
50
PCO
HDC + PCO
40
30
20
10
pH =3.4
0
−rTOC,0 (mg L−1 min−1
)
r2
−rTOC,0 (mg L−1 min−1
)
r2
pH =2.9
f
2,4-D
MCPA
4-CP
0.23
0.25
0.10
0.956
0.980
0.990
0.24
0.24
0.06
0.967
0.989
0.992
The initial photodegradation rate for both phenoxy pesticides was
almost the same, despite the fact that 2,4-D presents chlorine atoms
that are more accessible to oxidation [39], it was photodegraded
at lower irradiation times than MCPA, due to the presence of the
methyl group, an electron donor that may increase the dehaloge-
nation rate of haloaromatic pesticides [16]. On the other hand, the
value obtained for the initial TOC photodegradation rate of 4-CP
was half that of the chlorinated herbicides, probably as a result
of the different mechanisms involved, i.e. h+ hole-mediated in the
phenoxy pesticides, and •OH radical-mediated in the case of the
chlorophenols [16,37].
Taking into consideration that the 2,4-D and MCPA aqueous
solutions consisted mainly of their anionic species due to the disso-
ciation constant values (pKa 2,4-D = 2.7 and pKa MCPA = 3.1), under
these conditions, where the TiO2 photocatalyst was suspended in
the acidic solution, the surface became positively charged, because
of the isoelectric point of TiO2 P25 (pHPIE TiO2 P25 = 6.3), which
facilitates herbicide–photocatalyst interactions [40].
Regarding the behavior of 4-CP which has a pKa value of 9.41, it
was poorly dissociated in aqueous solution as confirmed by the ini-
tial pH value of 6.5. Under these conditions the TiO2 photocatalyst
surface is neutral and the herbicide–photocatalyst interactions are
weak electrostatic forces [15], which slows down the photodegra-
dation as demonstrated by the r0,TOC value.
Fig. 2 shows the evolution of chloride during the PCO process
for the three pesticides studied. The efficiency of chloride release
in the phenoxy pesticides was practically complete for 2,4-D and
MCPA, whereas a lower formation was observed for 4-CP. More-
over, the fact that almost all the chlorine corresponding to the
initial concentration of both chlorinated phenoxy herbicides was in
the form of chloride ions indicates the almost complete absence of
chlorinated intermediates after 360 min irradiation time, and this
was confirmed by the almost complete TOC removal of phenoxy
herbicides. On the contrary, in the 4-CP, chlorinated by-products
were not-detected [36], and this may have resulted in the residual
organic matter observed, because only 82% chloride formation and
87% TOC conversion, were obtained at the end of the photocatalytic
reaction.
0
B
TOC
TOC MCPA
TOC LMWCA
50
40
30
20
10
pH =3.6
0
pH =1.7
f
0
100
75
50
25
0
C
TOC
TOC 4-CP
Ecotoxicity
50
40
30
20
10
0
TOC 4-chlorocatechol
TOC benzoquinone
TOC LMWCA
pH = 6.5
0
pH = 3.3
f
0
50
100
150
200
250
300
350
Time (min)
Fig. 1. Time-evolution of organochlorinated compounds, aromatic intermediates,
and LMWCA expressed as TOC, during photodegradation of 2,4-D (A), MCPA (B) and
4-CP (C). (C) also includes the evolution of ecotoxicity during 4-CP photodegradation.
by-products were detected [21]. Total conversions of 2,4-D and the
2,4-dichlorophenol aromatic intermediate rose after 360 min reac-
tion time. With respect to 4-CP photodegradation, 4-chlorocathecol
and benzoquinone were the main aromatic intermediates detected,
but in very low concentrations. Moreover, in all cases, the LMWCA
detected were minority acetic, maleic, oxalic and formic acids.
Hydrochloric acid was also observed as a consequence of the C Cl
bond rupture.
It is well known that the photocatalytic degradation of these
compounds is caused by an attack of surface holes generated and/or
hydroxyl radicals and depends on both the pH and irradiation
wavelength [34]. Taking into account that any adsorption phe-
nomenon was observed with these chlorinated pollutants, both
during the initial dark period and during the photocatalytic process,
under the operating conditions used in this work, it would seem
likely that the major pathways of photocatalytic oxidation of phe-
noxy pesticides are through homolysis of the carbon–oxygen bond
on its aromatic ring [35–37]. This step is very fast and leads to higher
disappearance rates for 2,4-D and MCPA than for chlorophenols
which is illustrated by the preceding figures.
1.0
0.8
0.6
0.4
PCO
2,4-D
0.2
MCPA
4-CP
0.0
0
50
100
150
200
250
300
350
Time (min)
The initial TOC photodegradation rates for the three pollutants
used have been calculated [38], and are summarized in Table 1.
Fig. 2. Time-evolution of chloride/chlorine initial ratio during upon photocatalytic
oxidation.
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(A)
(B)
120
HDC
phenoxyacetic acid
2-chlorophenoxyacetic acid
4-chlorophenoxyacetic acid
2,4-D
TOC
TOC LMWCA
pH0 = 3.4
pHf = 2.9
PCO
pH0 = 2.9
pHf = 3.1
100
80
60
40
20
0
0
20
40
60
80
0
50
100
150
200
250
300
350
Time (min)
Fig. 3. Time-evolution of 2,4-D, 2-chlorophenoxyacetic, 4-chlorophenoxyacetic, phenoxyacetic acid and overall TOC concentration and TOC corresponding to LMWCA during
the sequential treatment of 2,4-D. (A) 1st step: HDC and (B) 2nd step: PCO.
3.2. Sequential treatment (hydrodechlorination + photocatalysis)
4-chlorophenoxyacetic acid was detected at concentrations lower
than 0.1 mM. Total dechlorination was achieved at 90 min reaction
time, and phenoxyacetic acid was the only organic reaction prod-
uct detected at that time. The hydrochloric acid produced during
the HDC process, did not have an important influence on the vari-
ation of the effluent pH. The acidic HDC-effluent was subsequently
treated by heterogeneous photocatalysis. Some residual organic
matter was detected at the end of the photocatalytic process given
that 92% TOC conversion was found. The absence of aromatic inter-
mediates in the reaction media can be explained by the LMWCA
(formic, acetic and maleic) plus corresponding phenoxyacetic acid
detected were responsible for that residual TOC.
Fig. 4 shows the sequential treatment for MCPA. During the HDC
reaction (Fig. 4A) 2-methylphenoxyacetic acid and hydrochloric
acid were the only reaction products detected under the operat-
ing conditions used. The total conversion of MCPA was achieved
at 30 min reaction time, no important changes in effluent pH were
detected during the experiment. The resultant acidic effluent from
All the chlorinated pesticides studied were treated in a sequen-
tial process, where the effluent obtained after a first step, based on
HDC with Pd/Al2O3, was treated by PCO in a second step with TiO2
P25. This was carried out in order to analyze whether the previ-
ous removal of chlorine from HDC in the initial organochlorinated
compounds resulted in an improvement in the final efficiency of
the photocatalytic process. In all cases, the HDC reaction provoked
a transformation of organochlorinated compounds. C and Cl bal-
ance closed in more than 97 and 99%, respectively, indicating that
the reactants and products were properly quantified in this step.
The sequential treatment of 2,4-D removal is represented in
Fig. 3, where 2-chlorophenoxyacetic, 4-chlorophenoxyacetic and
phenoxyacetic acids were identified as the organic reaction prod-
ucts during 2,4-D HDC process. According to the evolution of the
concentration of the products, the HDC of 2,4-D occurred faster
at the para than at the ortho position [16,39] and consequently,
(A)
(B)
100
HDC
pH = 3.5
0
PCO
2-Methylphenoxyacetic acid
2-Methyl-4-chlorophenoxyacetic acid
pH = 3.2
pH = 3.2
f
o
TOC
TOC LMWCA
80
60
40
20
0
pH = 3.5
f
0
20
40
60
80
0
50
100
150
200
250
300
350
Time (min)
Fig. 4. Time-evolution of MCPA, 2-methylphenoxyacetic acid and overall TOC concentration and TOC corresponding to LMWCA during the sequential treatment of MCPA.
(A) 1st step: HDC and (B) 2nd step: PCO.
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5
(A)
(B)
100
100
75
50
25
0
pHo= 6.7
pHf= 3.2
HDC
PCO
Ciclohexanone
Phenol
4-CP
TOC
TOC LMWCA
Ecotoxicity
80
pH = 3.2
o
pH = 3.2
f
60
40
20
0
0
20
40
60
80
0
50
100
150
200
250
300
350
Time (min)
Fig. 5. Time-evolution of 4-CP, phenol, cyclohexanone and overall TOC concentration and TOC corresponding to LMWCA during the sequential treatment of 4-CP. (A) 1st
step: HDC and (B) 2nd step: PCO. (B) also includes the evolution of ecotoxicity versus time during the 2nd step of the sequential treatment.
catalytic HDC was treated by PCO (Fig. 4B); at the end of the
process (360 min) 83% TOC conversion and 95% conversion for 2-
methylphenoxyacetic acid was achieved, which indicates the exis-
tence of some non-identified intermediate organic compounds in
addition to the LMWCA (formic, acetic and maleic acids) detected.
According to previous results, the phenoxyacetic herbicides
(2,4-D and MCPA) were efficiently removed by both the single PCO
treatment and the combined process, with an identical initial TOC
rate (Table 1) and practically complete mineralization. The effluents
from 2,4-D and MCPA after HDC were composed of hydrochloric
aqueous solutions of phenoxyacetic and 2-methylphenoxyacetic
acids, respectively. These were almost completely dissociated due
to their dissociation constant values (pKa phenoxyacetic acid = 3.12
and pKa 2-methylphenoxyacetic acid = 3.23). Under these con-
ditions, as was observed during the single process, the TiO2
photocatalyst surface suspended in the acidic solution became
positively charged and phenoxyacetate–photocatalyst interactions
were enhanced [41]. The lower TOC conversion achieved by the
combined process could be related to the absence of chlorine in the
organic molecule that could have had a synergic effect during the
photocatalytic degradation [42,43].
The sequential treatment of 4-CP (Fig. 5) shows that the HDC of
4-CP (Fig. 5A) leads to the formation of phenol and cyclohexanonein
addition to hydrochloric acid, which caused an important decrease
in the pH of the reaction medium. After 45 min operation some
of the phenol concentration was transformed to cyclohexanone.
Consequently, at the total conversion of 4-CP, the organic content
of the acidic effluent transferred to the photocatalytic stage was
composed of 94% phenol and 6% cyclohexanone (Fig. 5B). At the
beginning of the PCO stage, cyclohexanone was totally removed,
but a low concentration of phenol remained at the end with 72%
TOC removal. Longer irradiation times are well known to lead to
total phenol mineralization by photocatalysis with TiO2 P25 [44].
Analogously to 4-CP PCO, the initial compounds fed to the
second PCO stage are found in the non-dissociated phenol and
cyclohexanone forms that do not have any affinity to the TiO2 pos-
itively charged surface (because of the hydrochloric acid present in
the after-HDC effluent) and the combined degradation was r0,TOC
which is even worse than for the single PCO.
These figures also show that the complete chlorine removal for
2,4-D, MCPA and 4-CP compounds was achieved at 90 min. A com-
parison of the initial chloride formation rates (Table 2) between
both the single and the combined processes reveal significant dif-
ferences, with values showing rates differing by at least one order
of magnitude.
3.3. Ecotoxicity
The ecotoxicity values of the final effluents were measured and
compared with those of the initial solutions. Table 3 shows that the
photodegradation of 2,4-D and MCPA did not result in an increase
in the effluent ecotoxicity. Although a small quantity of oxidation
products remained after 6 h, these by-products only corresponded
to the LMWCA detected in the aqueous solution, without signif-
icance in terms of toxicity. In the case of 4-CP photodegration, the
fact that no total organic matter was removed by the end of the
photocatalytic process means that the treated effluent was signifi-
cant in terms of its ecotoxicity. The evolution of ecotoxicity values
versus irradiation time for the photodegradation of 4-CP was also
studied (Fig. 1C). This figures shows that at the initial time the
resulting effluent was more toxic than the starting solution. This
could be related to the presence of low concentrations of benzo-
quinone in the reaction effluent, a compound characterized by high
1.0
0.8
0.6
0.4
HDC
2,4-D
MCPA
4-CP
0.2
0.0
0
20
40
60
80
Fig. 6 shows a general view of the dechlorination capacity,
during the HDC reaction in the combined process for the three
herbicides studied. Fig. 6 can be compared with Fig. 2 which shows
the chlorine release during a single photocatalytic degradation.
Time (min)
Fig. 6. Time-evolution of chloride/chlorine initial ratio during the first step (HDC
reaction) of the sequential treatment.
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Table 2
Initial rate of chloride formation for 2,4-D, MCPA and 4-CP in the single photodegradation and HDC stage of sequential process.
PCO
rCl
HDC + PCO
(mmol L−1 min−1
−
(mmol L−1 min−1
)
r2
rCl
−
)
r2
,0
,0
2,4-D
MCPA
4-CP
3.4 × 10−3
2.0 × 10−4
1.7 × 10−3
0.998
0.998
0.999
4.6 × 10−2
5.5 × 10−2
6.1 × 10−2
0.988
0.999
0.998
Table 3
Ecotoxicity measurements in the single photocatalysis (PCO) and sequential process: HDC + photocatalysis (PCO).
PCO
HDC + PCO
Initial UT
Final UTPCO
After HDC UTHDC
Final UTHDC+PCO
2,4-D
MCPA
4-CP
0.7
1.1
51
0.4
<0.02
11
0.2
0.4
2.5
2.5
4.1
24
ecotoxicity (EC50 < 0.01 mg L−1). The evolution of this intermediate
during the reaction is consistent with the shape of the ecotoxicity
curve that revealed a maximum at intermediate times [45].
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