Accepted Manuscript Photocatalytic-assisted ozone degradation of metolachlor aqueous solution C.A. Orge, M.F.R. Pereira, J.L. Faria PII: DOI: Reference:
S1385-8947(16)30943-3 http://dx.doi.org/10.1016/j.cej.2016.06.136 CEJ 15446
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Chemical Engineering Journal
Please cite this article as: C.A. Orge, M.F.R. Pereira, J.L. Faria, Photocatalytic-assisted ozone degradation of metolachlor aqueous solution, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej. 2016.06.136
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Photocatalytic-assisted ozone degradation of metolachlor aqueous solution
C.A. Orge*, M.F.R. Pereira, J.L. Faria Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM) Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal.
[email protected],
[email protected],
[email protected]
* Corresponding author: Carla A. Orge e-mail:
[email protected] tel: +351225081400 fax: +351225081440
1
Abstract
2
Photocatalytic-assisted ozone degradation of metolachlor (MTLC) aqueous solutions
3
was investigated using neat TiO2 (prepared by sol-gel method) and TiO2/carbon
4
composite (prepared from commercial available metal oxide and carbon phase) as
5
catalysts.
6
In terms of MTLC degradation, O3 on its own is enough to achieve 100% removal, but
7
the introduction of light increased the rate of removal. On the other hand, the
8
combination of O3 with light and the tested catalysts is mandatory to reach high
9
mineralization in short reaction times. After 60 min of reaction, the TOC removal was
10
87% and 75% in the presence of the prepared composite and TiO2, respectively.
11
The concentration of two short chain carboxylic acids, oxalic and oxamic acids, was
12
followed during MTLC degradation. The amount of these acids decreased when O3 and
13
light were combined.
14
In general, nitrogen ions, such as nitrate and ammonium, were detected in the studied
15
processes. All treatments released ammonium and light based processes also
16
produced nitrate.
17
Microtox analysis showed that the combined process in the presence of the prepared
18
catalysts led to a remarkable reduction in the toxicity of the treated solution, decreasing
19
the inhibition of luminescent activity of Vibrio Fisheri from 74% to 12%.
®
20 21 22 23 24 25 26 27
Keywords: metolachlor, photocatalysis, ozonation, photocatalytic ozonation
1
1.
Introduction
2
The pollution caused by pesticides has significantly increased due to their extensive
3
use in agriculture. Their residues are known to be persistent in surface water and
4
ground water supplies posing a major problem in many countries [1]. Pesticides are
5
highly noxious, sometimes non-biodegradable and very mobile throughout the
6
environmental [2]. Chloroacetamides are the most widely used herbicides and their
7
operation mode consists in inhibiting the early development of susceptible weeds by
8
preventing biosynthesis of very long fatty acid chains, thus affecting cell integrity [3-5].
9
Among the most commonly used chloroacetamides are acetochlor, alachlor and
10
metolachlor (MTLC) [3, 6] and they have been detected in surface water and ground
11
water from 0.1 to 10 µg/L [7-10]. MTLC is listed in the Drinking Water Contaminant
12
Candidate List of the US Environmental Protection Agency [11]. MTLC and its
13
metabolites are alleged or confirmed carcinogens.
14
Many conventional water treatment processes, such as coagulation and chlorination,
15
have been found to be ineffective to remove alachlor and MTLC [12]. Therefore,
16
successfully elimination of pesticides in aqueous environment requires application of
17
unconventional processes [1, 2].
18
Sunlight is an efficient degradation pathway of MTLC in soil, with the drawback of
19
leaving behind some of its hazardous metabolites. It is estimated that about 50% of
20
applied MTLC degrades in eight days on sunlit soil [13]. The degree of
21
photodegradation diminishes rapidly with the deepening of soil incorporation. In water,
22
it undergoes mesolytic degradation to give primarily 4-(2-ethyl-6-methylphenyl)-5-
23
methyl-3-morpholine [14].
24
The photocatalytic degradation of MTLC using pure TiO2 and Ag modified TiO2 was
25
investigated by Sakkas et al. [15]. Working with low to moderate amounts of TiO2, they
26
concluded that in the early degradation steps the toxicity of the solution increases due
27
to the formation of more toxic by-products.
3
1
In the presence of organic matter the photodegradability of MTLC is hindered, while in
2
the presence of nitrate the opposite is observed, because the production of HO●
3
radicals is increased [16].
4
Simulated drinking water samples containing a mixture of chloroacetamide herbicides
5
and chloroacetamide derivatives were subjected to different treatments by Hladik et al.
6
[12]. Coagulation provided little removal of the parent and derivative compounds, but
7
chlorination was able to remove completely the by-products. However, ozonation
8
proved to be even more efficient than chlorination [12].
9
According to Munoz et al., homogeneous Fenton-like oxidation (H2O2/Fe3+) of
10
monochlorophenols is strongly dependent of the operating conditions [17]. The
11
stoichiometric amount of H2O2 and Fe3+ is the key to achieve suitable results.
12
The use of organic matter from compost to promote the photocatalytic degradation of
13
two herbicides and a fungicide by solar light was studied by Coelho et al. [18].
14
Following their findings the main conclusion was that hydroxyl radicals are the principal
15
species involved in the reactions, mainly due to their high reactivity.
16
On the other hand, Avetta et al. [19] proposed a different mechanism suggesting the
17
participation of singlet oxygen species in the photodegradation of monochlorophenols
18
in the presence of soluble bio-based substances (SBO). For different organic
19
substrates the toxicity assays showed a progressive, up to a complete, detoxification of
20
the system, mediated by the singlet oxygen species with no significant contribution of
21
the present SBO.
22
Even though a direct comparison of the efficiency of TiO2 and ZnO may not be precise
23
due to differences in their surface properties, Fennoll et al. verified that ZnO was more
24
efficient during photocatalytic degradation of a mixture of triazina and chloroacetanilide
25
herbicides [20].
26
Other studies reported the ozonation of MTLC, individually or in a mixture of emerging
27
pollutants, in semi-batch and continuous operation [21, 22]. In terms of toxicity, the by-
28
products released with O3 alone are more toxic than the parent compound, but the 4
1
addition of the carbon material reduces this impact. Multi-walled carbon nanotubes and
2
carbon nanofibers grown on a honeycomb cordierite were used as catalysts.
3
A study on the toxicity of photoproducts formed during UV-treatments of three
4
chloroactamide herbicides showed that 90% of the original pesticide was converted in
5
compounds with more or equal toxicity than the parent compound [3].
6
In summary, this study focuses on using photocatalytic-assisted ozone process for
7
MTLC degradation. To the best of our knowledge, the present work describes the
8
MTLC degradation using photocatalytic ozonation by the first time. For that purpose
9
two catalysts were prepared, one composite made of commercial TiO2 and multi-walled
10
carbon nanotubes and another consisting in TiO2 synthetized by the sol-gel procedure.
11
Experiments without catalysts and with only ozone and radiation were also performed
12
in order to better understand the results. The performance of the prepared materials
13
was compared with the commercial TiO2 (P25).
14 15 16
2.
Experimental
2.1 Reagents and materials
17
MTLC (C15H22ClNO2, PESTANAL Analytical Standard), nitric acid (HNO3, ≥65%), oxalic
18
acid (C2H2O4, ≥ 99%), oxamic acid (C2H3O3N, ≥98%), tert-butanol ((CH3)3OH, ≥99.5%),
19
titanium (IV) isopropoxide (Ti[OCH(CH3)2]4, 97%), 2-ethyl-6-methylaniline
20
(C2H5C6H3(CH3)NH2, ≥99.5%) and 2,6-pyridine dicarboxylic acid (C7H5NO4, 99%) were
21
purchased from Sigma-Aldrich. Sodium carbonate (Na2CO3, ≥ 99%) was obtained from
22
Fluka. Acetonitrile (C2H3N, HPLC gradient grade) and sulfuric acid (H2SO4, 95-98%)
23
was supplied by Fisher Scientific. Methanol (CH3OH, MS grade) was acquired from
24
VWR International. Ultrapure water was supplied by a Milli-Q water system.
25
Commercial TiO2, sample P25, was supplied by Evonik Degussa Corporation. The
26
commercial multi-walled carbon nanotubes, MWCNT, were supplied by Nanocyl (ref.
27
3100).
28
2.2 Kinetic experiments 5
1
Photocatalytic ozonation of MTLC was performed in a glass immersion photochemical
2
reactor according to the experimental conditions described in [23]. The initial
3
concentration of MTLC was 20 ppm and the reactor was loaded with 0.5 g L-1 of
4
catalyst. The reaction system was the same for all tested processes; however, in the
5
case of photolytic reactions the ozone was replaced by oxygen and for ozonation
6
experiments the radiation source was turned off.
7
In the experiments carried out with tert-butanol, the radical scavenger is presented in
8
excess with a concentration 10 times higher than the initial concentration of the parent
9
compound (Ctert-butanol = 0.7 mmol L-1).
10
For this study, different catalysts were tested in the kinetic reactions. Recent
11
researches of our group have confirmed that composites based on TiO2 and carbon
12
nanotubes present high catalytic activity in the photocatalytic oxidative degradation of
13
several pollutants [23-27]. The sol-gel method has been widely applied to prepare TiO2
14
photocatalysts to be used in the development of prototypes at laboratorial scale [28-
15
30]. Composite of 90:10 (w/w) P25 and MWCNT was synthetized by the hydration-
16
dehydration technique, sample P2590MWCNT10 [25, 27]. In the composite preparation,
17
a selected amount of MWCNT was dispersed in water under ultrasonication. P25 was
18
added to the suspension 30 min later and the mixture was heated up to 80 ºC and
19
magnetically stirred until the water was completely evaporated. The resulting
20
composite was dried at 110 ºC overnight. TiO2 sample was obtained by the sol-gel
21
technique [30]. The preparation consisted in the slowly addition of Ti[OCH(CH3)2]4 to
22
ethanol. After 30 min under continuous stirring, nitric acid was added. The solution was
23
loosely covered and kept stirring until the homogeneous gel formed. After grinding the
24
xerogel, a fine powder was obtained and afterwards it was calcined at 400 ºC in a
25
nitrogen flow for 2 h.
26
The textural characterization of the materials was obtained from the N2 equilibrium
27
adsorption/desorption isotherms, determined at -196 °C with a Quantachrome
28
Instruments NOVA 4200e apparatus. The relative amount of TiO2 in the composite was 6
1
determined by thermogravimetric analysis (TG) under air in a STA 409 PC/4/H Luxx
2
Netzsch thermal analyser. Detailed results of characterization were reported in a
3
previous work [23].
4 5
2.3 Analytical techniques
6
The MTLC concentration was monitored by HPLC, using a Hitachi Elite LaChrom
7
device fitted with a diode array detector (DAD). The separation of the pollutant was
8
attained using a Lichrocart C18-RP Puroshper Star (250 mm × 4.6 mm, 5 µm) column
9
with an isocratic mobile phase containing 60% of acetonitrile and 40% of water. The
10
concentration of short-chain carboxylic acids resulted from MTLC degradation, oxalic
11
acid (OXA) and oxamic acid (OMA), was followed by a Hitachi Elite LaChrom HPLC
12
provided with an UV-Vis detector and an Alltech OA-1000 chromatography column
13
operating with an isocratic mobile phase of 5 mmol L-1 H2SO4.
14
Analyses with an Ultra High Performance Liquid Chromatography with tandem Mass
15
Spectrometry (UHPLC–MS/MS) were carried out to verify the presence of some MTLC
16
intermediates reported in the literature. For UHPLC–MS/MS analyses, a Shimadzu
17
Corporation apparatus (Tokyo, Japan) was used, consisting of a Nexera UHPLC
18
equipment, coupled to a LCMS-8040 triple quadrupole mass spectrometer detector
19
with an electrospray ionization source operating in both positive and negative ionization
20
modes.
21
(Phenomenex, Inc., California, USA) was operated under reversed mode with a mobile
22
phase consisting of a mixture of methanol and water (70/30, v/v) with a flow rate of
23
0.22 mL min-1, temperature of 20 ºC and a volume of injection of 20 µL. The details of
24
operation mode of UHPLC-MS/MS are reported in [31].
25
In order to evaluate the mineralization degree of the processes, the total organic
26
carbon (TOC) was obtained with a Shimadzu TOC-5000A apparatus.
The
column
Kinetex™
1.7 µm
XB-C18
100 Å
(100 × 2.1 mm
i.d.)
7
1
The ions formed during MTLC degradation were accomplished by the technique of ion
2
chromatography by a MetrOHM 881 Compact IC Pro with 863 Compact Autosampler
3
as described in a previous work [32].
4
Microtox
5
ISO/DIS 11348-3 [33], were performed with the purpose of evaluating the toxicity
6
caused by compounds released during the MTLC degradation. The experimental
7
details are reported in [34]. Summarising, this procedure measures the inhibition of the
8
light emission of bioluminescent bacteria (V. fischeri) promoted by the toxic effect of the
9
tested chemicals, during 30 min of incubation at 15 °C.
®
acute toxicity analyses, the procedure of which is described by standard
10 11 12
3.
Results 3.1 MTLD degradation
13
The experimental results corresponding to MTLC degradation during single ozonation
14
(O3), photolysis (Light), photolytic ozonation (PO3) and photocatalytic ozonation (PCO3)
15
corresponding to MTLC degradation are depicted in Figure 1.
16
In order to better evaluate the performance of the different processes, the data
17
corresponding to MTLC decay were fitted during the first 15 min of reaction.
18
The PCO3 follows a pseudo-first-order kinetic rate model. The PCO3 of MTLC can be
19
described by a Langmuir-Hinshelwood kinetic model [35] and assuming that due to
20
combined effects of light and O3 the concentration of HO● with respect to MTLC is
21
quasi constant.
22
According to this model, in the absence of any solid catalyst, the evolution of MTLC
23
concentration during the oxidation process was found to be well described by the
24
following equation:
−
dCେ = k ୦୭୫ Cେ dt
(1)
8
1
where khom (min-1) represents the first-order apparent rate constant and CMTLC (mmol L-1)
2
is the concentration of the MTLC in each instant. Integration of Eq. (1), considering
3
CMTLC = CMTLC,0, when t = 0, leads to:
ln
(2)
Cେ, = k ୦୭୫ t Cେ
4
When the prepared materials are introduced, both homogeneous and heterogeneous
5
degradation occur. Therefore, the MTLC removal rate is the sum of the two
6
contributions:
−
(3)
dCେ = (k ୦୭୫ + k ୦ୣ୲ )Cେ dt
7
where khet (min-1) represents the first-order apparent rate constant for the
8
heterogeneous degradation. Integration of Eq. (3), considering kapp = khom + khet and
9
CMTLC = CMTLC,0, when t = 0, leads to:
ln
(4)
Cେ, = k ୟ୮୮ t Cେ
10
The corresponding rate constants are given in the Table 1.
11
Table 1: First-order apparent rate constants of non-catalytic and catalytic runs of MTLC
12
decay.
13 14
System (Catalyst)
kapp×102 ± 25% (min-1)
O3 (-)
16*
Light (-)
4*
PO3 (-)
27*
PCO3 (P250.9MWCNT0.1)
28
PCO3 (TiO2)
12
PCO3 (P25)
36
(-) no catalyst; * homogeneous first-order rate constants.
15
Ozonation by itself allowed a fast decay of MTLC concentration and a complete
16
elimination was attained in 30 min. This is observed because activated aromatic rings
17
or double bonds, which are present in MTLC, are selectively attacked by O3. Since 9
1
MTLC is an aromatic compound, it presents a high delocalization of electrons and
2
exhibits enhanced reactivity towards O3. This behavior is in accordance with the
3
literature, which suggests that the degradation of MTLC is mainly due to the direct
4
reaction with ozone [21].
5
On the other hand, photolysis did not easily remove MTLC. Sakkas et al. also verified
6
that MTLC concentration in aqueous solutions did not decrease under direct irradiation
7
[15]. The presence of light was not enough to completely remove MTLC from the
8
solution in a short period of reaction.
9
The combination of O3 with near UV/Vis light led to a slightly faster decay of MTLC than
10
single ozonation (1.7 times higher).
11
In the case of PCO3, MTLC degradation depends on the type of catalyst. The
12
commercial sample (P25) presents the highest removal rate (kapp = 36×10-2 min-1),
13
leading to a total conversion after 5 min of reaction. When the prepared composite was
14
combined with O3 and light (kapp = 28×10-2 min-1) no significant improvement was
15
verified when compared to the non-catalytic-combined run (khom = 27×10-2 min-1), but
16
with the synthesized TiO2 a decrease in the degradation rate was observed. Although
17
the different catalysts did not present the same performances, total MTLC removal was
18
observed until 30 min of reaction with all O3 based processes, as expected [21].
19
In order to better understand the influence of HO● radicals in the reaction mechanism
20
of MTLC degradation, experiments were carried out in the presence tert-butanol, a well
21
known hydroxyl radical scavenger. Tert-butanol reacts very rapidly with hydroxyl
22
radicals (k = 5 x 108 M-1 s-1) [36] and very slowly with ozone ( k = 0.03 M-1 s-1) [37, 38].
23
These results are depicted in Figure 2.
24
The MTLC degradation was practically not affected by the addition of the radical
25
scanvenger tert-butanol during O3 based processes, both in the presence and absence
26
of catalyst or light. A very similar trend was observed by Restivo et al. [21]. This result
27
suggested that MTLC reacts mainly with O3 under the current experimental conditions.
10
1
In the case of photolysis, a significant influence was verified by the addition of tert-
2
butanol, which means that HO● radicals play a key role in the presence of light alone.
3
Benitez et al. also reported that MTLC photooxidation rate was negatively affected by
4
the presence of tert-butanol, independently of type of water [39].
5 3.2 TOC removal
6 7
In order to evaluate the efficiency of removing the by-products released during MTLC
8
degradation, analyses of TOC were carried out at 60, 120 and 180 min of reaction. The
9
results of normalized TOC content are depicted in Figure 3.
10
The non-catalytic individual methods, single ozonation and photolysis, present a low
11
TOC removal, less than 20% after 180 min of reaction. This occurs because the by-
12
products of the oxidation of MTLC are less reactive than MTLC to O3.This behavior is in
13
accordance with what was previous reported [21, 40], only a slow decrease of organic
14
matter and no variation was observed after 8 h during photolysis under Xe irradiation.
15
On the other, when these two processes were combined the TOC content significantly
16
decreased. O3 and light together removed 40% of TOC after 60 min of reaction and the
17
amount of TOC removed doubled after 120 min of reaction; however, its value remains
18
in the last hour of the reaction. Independently of the catalyst introduced, when the
19
prepared samples were combined with near UV/Vis light, a significant amount of
20
organic matter was quickly removed. The best performance in terms of mineralization
21
degree was verified with the prepared composite, leading to 87% of TOC removal after
22
60 min of reaction. This value remained practically unchanged until the end of reaction
23
(91% of TOC removal after 3 h). In the case of TiO2 sample, a TOC depletion of 75%
24
and 85% was verified in the first and third hour of reaction, respectively. The
25
commercial catalyst presented a similar profile.
26 27
3.3
Individual methods
11
1
With the aim of evaluating the presence of synergetic effects during PCO3 with the
2
prepared materials, catalytic ozonation (CO3) and photocatalysis (PC) were individually
3
carried out. Adsorption experiments (ADS) were also performed. The results of
4
normalized MTLC concentration and TOC content are presented in Figure 4.
5
Both CO3 and PC in the presence of prepared samples led to a fast decay of MTLC. In
6
terms of TOC removal, PC presents better results than CO3. In the case of the
7
composite during PC, a high content of TOC was removed after 120 min of reaction
8
(84%). This value is very similar to that obtained by PCO3 (90%), although the
9
combined method achieved a higher removal after only 1 h of reaction. PC with TiO2
10
removed approximately 50% of organic matter. The prepared composite had a higher
11
adsorption capacity than the neat TiO2. This improvement can be attributed to the
12
presence of the carbon phase in the composite.
13
Summarizing, CO3 and PC in the presence of prepared samples are effective in MTLC
14
degradation; however, high mineralization rates are only accomplished in a short
15
period of reaction by PCO3.
16 17
3.4
By-products analysis
18
Short chain carboxylic acids are final intermediates of a wide range of organic
19
compounds. Thus, the formation of OXA and OMA was followed during MTLC
20
degradation (Figure 5).
21
In the case of photolysis none of the acids was detected.
22
During single ozonation the concentration of OXA increases all the time, as expected
23
since OXA is refractory to O3 alone [41, 42]. PO3 produced the highest amount of OXA,
24
however at the end of the reaction all acid was degraded. This result was expected
25
since O3 and light together are able to remove OXA from the solutions [32]. In the
26
remaining processes only a small content was verified until 120 min of reaction.
27
As happens with OXA, the profile concentration of OMA depends on the applied
28
process. Analysing the results of the individual methods, photolysis did not produce 12
1
OMA and single ozonation released a slight amount after 120 min of MTLC
2
degradation. In the case of PO3, its concentration increases until approximately 0.030
3
mmol L-1 at 120 min of reaction and after that it starts to decrease. PCO3 in the
4
presence of TiO2 released the highest amount of OMA (approximately 0.100 mmol L-1),
5
but it disappeared in 1 h. In PCO3 with P250.9MWCNT0.1 only a small amount of OMA
6
was observed in the first hour of reaction. This decrease in OMA concentration is
7
expected in view of previous results because PCO3 with an appropriate catalyst can
8
eliminate OMA [23].
9
In order to evaluate the concentration of ions released during MTLC degradation, ion
10
chromatography analyses were carried out at specific times of reaction. Table 2
11
presents the concentration of ions and the nitrogen-balance of identified species at the
12
end of the reaction for all tested processes.
13 14
Table 2: Concentration of ions and N-balance of identified species (NT) at 180 min of
15
reaction. System (Catalyst)
16
NO3-
NH4+ -1
OMA -1
NT -1
(mmol L )
(mmol L )
(mmol L )
(mmol L-1)
O3 (-)
-
0.015
0.008
0.023
Light (-)
0.030
0.012
-
0.042
PO3 (-)
0.014
0.015
0.022
0.052
PCO3 (P250.9MWCNT0.1)
-
0.035
0.001
0.037
PCO3 (TiO2)
-
-
-
-
PCO3 (P25)
0.020
0.035
-
0.055
(-) no catalyst.
17
Analysing the ions formed in the non-catalytic methods, NH4+ was identified in all
18
treatments and UV-based processes also produced NO3-.
19
In the PCO3 in the presence of TiO2, N species were not detected at 180 min of
20
reaction. On the other hand, the prepared composite led to the formation of NH4+ and a
21
slight amount of OMA.
13
1
PO3 and PCO3 with commercial sample, P25, presented the highest amount of
2
identified species (Nmax = 0.07 mmol L-1).
3
The presence of non-identified nitrogen containing species, especially in the cases of
4
O3 alone and PCO3 with TiO2 must be taken into account. The adsorption of the parent
5
compound and the possible adsorption of nitrogenated oxidation by-products may also
6
be taken in account for the TiO2 material. In addition, the degradation of MTLC could
7
lead to the formation of nitrogen compounds in gas phase, such as N2 or nitrogen
8
oxides.
9
With the aim of evaluating the presence of other intermediates during MTLC
10
degradation, UHPLC-MS/MS analyses were carried out. Only the presence of 2-ethyl-
11
6-methylaniline was confirmed during photolysis at 120 and 180 min of reaction under
12
the experimental conditions used. This compound is a known intermediated from the
13
oxidation of MTLC and it is characterized by high toxicity [15, 43]. The chromatographic
14
analysis suggests that the concentration of 2-ethyl-6-methylaniline decreased after
15
120 min till 180 min. The presence of this intermediate during photolysis at the end of
16
reaction is in accordance with the remaining results. Since MTLC was slowly degraded
17
during photolysis, it is expected that the primary intermediates, as 2-ethyl-6-
18
methylaniline, are detected at this time of reaction, instead of final by-products as short
19
chain carboxylic acids, which are verified in the remaining processes.
20
Non observation of this compound during the remaining processes suggested that the
21
reaction mechanism is different from that of the O3 based processes. When the
22
reaction was carried only with light, 2-ethyl-6-methylaniline was detected; when O3 was
23
used alone or combined, other primary by-products were formed (their identification
24
was not possible at this stage with the existing analytical methods, due to their trace
25
concentrations).
26
3.5
27
In order to assess the toxicity caused by compounds produced during MTLC oxidation,
28
the acute toxicity of the untreated MTLC solution and solutions submitted to different
Bio-toxicity of MTLC degradation products
14
®
1
treatments during 180 min was evaluated by Microtox
bioassays. The marine
2
bacterium used to study the toxic substances was Vibrio fischeri. The results presented
3
in Figure 6 were obtained after 30 min of exposure by determination of inhibition
4
percentage in the bacteria luminescence caused by each sample.
5
On all tested processes, the final solution has less toxicity than the untreated solution,
6
even when the non-catalytic individual methods are applied. The toxicity observed
7
during MTLC photolysis can be attributed to the formation of 2-ethyl-6-methylaniline
8
that was identified during UHPLC-MS/MS analyses [15, 16].
9
According to the results, both non-catalytic and catalytic combined methods in the
10
presence of the prepared samples presented the most pronounced decrease.
11
Concerning the PCO3 systems that use the prepared catalysts (excluding the neat P25)
12
the measured percent of initial inhibition is 12%, representing an abatement of 84%
13
with relation to the value of the non-treated sample. PCO3 with the neat commercial
14
sample (P25) was less efficient than with the prepared catalyst. In addition, PO3 also
15
led to a treated solution with less toxicity than the catalytic reaction with P25. This
16
difference in the toxicity of the final solutions suggested that the introduction of a
17
carbon phase in the composite, as well as the presence of different crystalline phases
18
on TiO2 changed the paths of the reaction mechanism, and consequently the final by-
19
products formed.
20 21
4. Conclusions
22
The present work focuses on the photocatalytic-assisted ozone degradation of MTLC
23
aqueous solutions and reports this combined method for the first time. Two different
24
catalysts were used, neat TiO2 synthetized by the sol-gel method and TiO2/carbon-
25
nanotubes composite made of commercial TiO2 and multi-walled carbon nanotubes.
26
The degradation of MTLC molecule can be easily achieved by O3 alone. However, this
27
is not enough to achieve a high mineralization and a toxicity abatement. Indeed,
15
1
considerable amounts of organic matter are still present in solution after 60 min of
2
reaction in the absence of a (photo)catalyst. Additionally, the application of the
3
combined process using the prepared materials leads to a significant decrease in the
4
inhibition of the luminescent activity of Vibrio Fisheri.
5
The introduction of a carbon phase in the composite increased the TOC removal in
6
comparison with the parent TiO2, removing 90% of organics after 60 min of reaction.
7
The best toxicity reduction was achieved by PCO3 system that used the prepared
8
catalysts (not the commercial one), reaching 84% of abatement on the percent of initial
9
inhibition effect, after 180 min of reaction.
10
The PCO3 system was demonstrated to have great potential for MTLC degradation,
11
since the pollutant was removed very fast, a high mineralization degree was easily
12
reached and the final solution had low toxicity.
13 14
Acknowledgements
15
This work was financed by FCT and FEDER through COMPETE 2020 (Project
16
UID/EQU/50020/2013 - POCI-01-0145-FEDER-006984). C. A. Orge acknowledges the
17
research fellowship BPD/90309/2012 received from FCT. The authors are grateful to
18
Ana R. Ribeiro for UHPLC-MS/MS analyses, as well as the appropriate interpretation of
19
the results.
20 21
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1
Figure captions
2
Figure 1: Normalized concentration (C/C0) of MTLC in aqueous solution as a function
3
of time for O3, Light, PO3 and PCO3.
4
Figure 2: Influence of tert-butanol on the dimensionless MTLC concentration during
5
O3, Light, PO3 and PCO3 with prepared composite.
6
Figure 3: Normalized TOC content (TOC/TOC0) for MTLC in aqueous solution as a
7
function of time for O3, Light, PO3 and PCO3.
8
Figure 2: Normalized MTLC concentration (C/C0) and TOC content (TOC/TOC0) as a
9
function of time for CO3, PC and ADS experiments in the presence of P2590MWCNT10
10
(a) and (b) and TiO2 (c) and (d).
11
Figure 3: OXA (a) and OMA (b) concentration for single ozonation (O3), photolysis
12
(Light), PO3 and PCO3.
13
Figure 6: Results of Microtox tests at 180 min of reaction, with exposure time at
14
bacteria Vibrio fischeri of 30 min.
®
15 16 17
22
1.0
O3 Light PO3
0.8
PCO3/P2590MWCNT10 PCO3/TiO2
0.6 C/C0
PCO3/P25
0.4 0.2 0.0 0
1 2
10
20
30 t/min
40
50
60
Figure 4
3 4 5 6 7
23
1.0 Light O3
0.8
PO3 PCO3/P2590MWCNT10
C/C0
0.6 0.4 0.2 0.0 0 1 2
10
20
30 t/min
40
50
60
Figure 2
3 4 5 6
24
1.0
TOC/TOC0
0.8
O3 Light PO3
0.6
PCO3/P2590MWCNT10 PCO3/TiO2 PCO3/P25
0.4 0.2 0.0
0
1 2
20
40
60
80 100 120 140 160 180 t/min
Figure 3
3 4 5 6
25
1 1.0
1.0
ADS CO3 PC PCO3
0.8
0.8
TOC/TOC0
0.6
C/C0
0.6
0.4
0.4
(a)
0.2 0.0
0
20
40
60
80 100 120 140 160 180
0
20
40
60
ADS CO3 PC PCO3
0.8 0.6
1.0 0.8
C/C0
0.6
0.4
(c)
0.2 0.0
80 100 120 140 160 180 t/min
TOC/TOC0
1.0
4
0.0
t/min
2
3
(b)
0.2
0.4
(d)
0.2 0
20
40
60
80 100 120 140 160 180 t/min
0.0
0
20
40
60
80 100 120 140 160 180 t/min
Figure 4
5 6 7 8
26
0.100
a)
COXA (mmol L-1)
0.075
0.050
0.025
O3
0.000 0
20
40
60
80
100
120
140
160
180
t/min
Light PO3 PCO3/P2590MWCNT10 PCO3/TiO2
0.100
0.075
COMA (mmol L-1)
PCO3/P25
b)
0.050
0.025
0.000 0
20
40
60
80
100
120
140
160
180
t/min
Figure 5
1 2 3 4 5
27
1 2
100
3 4 5 6
% Inhibition effect
80 PCO3
60
40
7
20 8 9 10
0
Initial
O3
Light
PO3 P2590MWCNT10 TiO2
P25
Figure 6
11 12 13 14
28
Research Highlights
1 2
• O3 assisted photocatalytic degradation of metolachlor was investigated
3
• O3 by itself is enough to achieve total removal
4
• High mineralization was only easily attained with O3, light and tested samples
5
• The combined method led a pronounced decrease in the toxicity of the
6 7 8
solutions
• Prepared cataysts presented remarkable performance during metolachlor degradation
9 10 11 12 13
29
100
80
60
% 40
TiO2/MWCNT composite
near UV/Vis. radiation
O3
20
Inhibition
0 TOCremoval MTLCremoval
luminescent activity
@180 min
@60 min
@10 min
