International Journal of PhotoenergyVolume 2012 (2012), Article ID 371714, 11 pagesdoi:10.1155/2012/371714

Research Article

Photocatalytic Degradation of Pesticides in Natural Water: Effect of Hydrogen Peroxide

Natividad Miguel, María P. Ormad, Rosa Mosteo, and José L. Ovelleiro

Department of Chemical Engineering and Environmental Technologies, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza , Spain

Received 12 August 2011; Revised 20 November 2011; Accepted 1 December 2011

Academic Editor: Jae Sung Lee

Copyright © 2012 Natividad Miguel et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The aim of this paper is to evaluate the effectiveness of photocatalytic treatment with titanium dioxide in the degradation of 44 organic pesticides analyzed systematically in the Ebro river basin (Spain). The effect of the addition of hydrogen peroxide in this treatment is studied, and a monitoring of effectiveness of photocatalytic processes is carried out by measurements of physical-chemical parameters of water. The application of photocatalytic treatment with 1 g L⁻¹ of TiO₂ during 30 minutes achieves an average degradation of the studied pesticides of 48%. Chlorine demand, toxicity, and dissolved organic carbon (DOC) concentration of water are reduced. If hydrogen peroxide is added with a concentration of 10 mM, the average degradation of pesticides increases up to 57%, although chlorine demand and toxicity of water increase while DOC concentration remains unchanged with this treatment. The application of either photocatalytic treatments does not produce variations in the physical-chemical parameters of water, such as pH, conductivity, colour, dissolved oxygen, and hardness. The pesticides which are best degraded by photocatalytic treatments are parathion methyl, chlorpyrifos, α-endosulphan, 3,4-dichloroaniline, 4-isopropylaniline, and dicofol while the worst degraded are HCHs, endosulphan-sulphate, heptachlors epoxide, and 4,4′-dichlorobenzophenone.

1. Introduction

During recent years, numerous organic substances considered to be hazardous have been detected in waters of Ebro river basin (Spain). These substances have been detected in surface and ground waters and they can be considered hazardous substances according to the current legislation because they are toxic, persistent, and bioaccumulative substances.

Pesticides, artificially synthesized substances used to fight pests and improve agricultural production, are especially problematic. These are monitored by the Pesticides Control Network (Ebro river basin), which systematically analyzes 44 organic pesticides in surface waters. These pesticides were selected because of their appearance in lists of hazardous substances and/or their high level of use in Spanish agriculture. The 44 pesticides analyzed in the Ebro river basin are alachlor, aldrin, ametryn, atrazine, chlorfenvinfos, chlorpyrifos, pp′-DDD, op′-DDE, op′-DDT, pp′-DDT, desethylatrazine, 3,4-dichloroaniline, 4,4′-dichlorobenzophenone, dicofol, dieldrin, dimethoate, diuron, α-endosulphan, endosulphan-sulphate, endrin, α-HCH, β-HCH, γ-HCH, δ-HCH, heptachlor, heptachlor epoxide A, heptachlor epoxide B, hexachlorobenzene, isodrin, 4-isopropylaniline, isoproturon, metholachlor, methoxychlor, molinate, parathion methyl, parathion ethyl, prometon, prometryn, propazine, simazine, terbuthylazine, terbutryn, tetradifon, and trifluralin.

Due to the presence of hazardous substances, both natural water used to supply the population and water which is to be reused have to be treated in water treatment plants to ensure that they comply with the minimum criteria established in the current legislation [1–3].

Water treatments consist of several operations which often do not achieve the removal of hazardous substances or entail other problems deriving from the treatment.

In the case of drinking water production, an important successful development is the possible formation of organic-halogenated compounds, potentially carcinogenic and chlorinated deriving from methane, as reaction products between chlorine (common disinfectant product) and the organic matter in water. The most abundant of these are trihalomethanes (THMs) whose concentration is limited by Spanish Royal Decree 140/2003 at to 100 μg L⁻¹.

The drawbacks in the use of compounds with chlorine, among others, have derived in the research of other alternative treatments for removing pollutants in water treatments. Among these, advanced oxidation processes (AOPs) are based on the generation of reactive oxygen species, which are highly reactive, nonselective, and do not generate toxic byproducts [4, 5]. These species are capable of degrading a substantial number of pollutants by radicalary mechanisms. The generation of these species can be carried out by processes with ozone, hydrogen peroxide, metallic catalysts, UV radiation, and so forth.

One of the most frequently investigated AOPs in recent years is the photocatalytic process. Photocatalysis entail the combination of radiation and catalyst. Both are necessary in order to achieve or accelerate a chemical reaction, and therefore photocatalysis can be defined as the “acceleration of a photoreaction by the presence of a catalyst.”

Catalysts used in these processes are semiconductor metallic oxides materials. The surface of semiconductor metallic oxides provides a place where oxidation-reduction reactions can be started by radiation. Semiconductors have bands associated with energy levels spaced between them. Photoexcitation with energy equal to or greater than that of the gap between the valence and conduction bands moves an electron from the valence band to the conduction band, generating a hole deficient in electrons. The oxidation of an adsorbed molecule can be produced in holes and simultaneously the reduction of another molecule can be produced in the opposite part of the catalyst.

Numerous semiconductor substances have been tried as catalyst in photocatalytic processes. Generally, it is known that titanium dioxide is one of the most effective. Moreover, it has a high stability and photoactivity, low cost, nontoxicity, and solubility. The activation of titanium dioxide is produced with radiation at  nm.

As a result of the application of photocatalytic processes based on titanium dioxide, the number of references in the literature in recent decades related to the degradation of toxic and hazardous substances in water can be counted in their thousands. As regards the group of pesticides under study, photocatalytic processes have been applied in several ways and there are many references to the degradation of such pesticides such as triazines [6, 7], organic-phosphorated pesticides [8–10], alachlor [11, 12], diuron [7, 13], and parathion methyl [14].

The addition of hydrogen peroxide to photocatalytic treatment with titanium dioxide can be used to increase the effectiveness of the latter process because, as hydrogen peroxide is a more powerful oxidant than oxygen, it generates a larger collection of electrons and this avoids the recombination of electron-hole pairs formed in the photocatalytic process. Hydrogen peroxide is an electron acceptor and it can react with electrons in the conduction band and generate radicals.

Similarly, there are several references concerning the removal of pesticides by photocatalytic treatments with hydrogen peroxide, such as the degradation of atrazine, isoproturon, alachlor and diuron [7], organic-phosphorated pesticides [9, 10], and other triazines [15].

However, the results show a big disparity with respect to the effect of hydrogen peroxide. It is sometimes beneficial while at other times it is damaging to the effectiveness of the process. This phenomenon, the effect of hydrogen peroxide in photocatalytic treatments, produces additional drawbacks when complex mixtures of pollutants are used. The damaging effect in the degradation rate is produced by a modification of the catalyst surface by hydrogen peroxide adsorption [16] and the inhibition of generated holes in the valence band and reaction with hydroxyl radicals [17]. When the effect of hydrogen peroxide is beneficial, a substantial consumption of hydrogen peroxide is sometimes necessary in order to obtain only a small increment of the degradation rate.

The aim of this research work is to evaluate the degradation of the 44 pesticides systematically analyzed in the Ebro river by photocatalytic treatment with titanium dioxide and to study the effect of hydrogen peroxide in this treatment. Moreover, a monitoring of the effectiveness of the photocatalytic processes is carried out by measuring the physical-chemical parameters of water.

2. Materials and Methods

2.1. Samples

Samples of natural water from the Ebro river upstream from Zaragoza (Spain) were prepared by fortification with 44 pesticides in concentrations of 500 ng L⁻¹. These 44 organic pesticides, listed in the introduction section, are systematically analysed in surface waters by the Network of Control of Pesticides in the Ebro river basin.

2.2. Sample Characterization

The characterization of the samples was carried out using the parameters shown in Table 1. The equipment used and standard methods applied are detailed below.

Table 1: Parameters, equipment, and standardized methods used for the characterization of samples.

Parameter
Equipment
Standard method
Reference

pH and temperature
pH-meter CRISON GLP21
SM 4500-HB
[18]

Conductivity
Conductivity-meter CRISON Basic30
UNE-EN 27888 : 1994
[19]

Dissolved organic carbon (DOC) concentration
Analyzer Shimadzu TOC-VCSH
SM 5310B
[18]

Chlorine Demand
—
SM 5710B
[18]

Turbidity
Turbidity-meter Hanna LP2000
ISO 7027: 1999
[20]

Color
Multiparametric Photometer Hanna C99
SM 2120B
[18]

Dissolved oxygen concentration
Multiparametric Photometer Hanna C99
SM 4500-O C
[18]

Hydrogen peroxide concentration
—
—
[21]

Hardness
Multiparametric Photometer Hanna C99
SM 2340B
[18]

Suspended solids concentration
Multiparametric Photometer Hach Lange DR2800
SM 2540D
[18]

Phosporous concentration
Multiparametric Photometer Hanna C99
SM 4500-P C
[18]

Ammonia concentration
Multiparametric Photometer Hanna C99
SM 4500-NH3 C
[18]

Cyanide concentration
Multiparametric Photometer Hanna C99
SM 4500-CN E
[18]

Fluoride, chloride, nitrate, phosphate, and sulphate concentration
Ionic Chromatographer DIONEX ICS-1000
SM 4110B
[18]

Toxicity
Toxicity-meter LUMIStox 300
ISO 11348: 1999
[22]

Pesticides
Gas chromatographer TRACE2000 and mass spectrometer POLARIS
EPA 525.2
[23]

The analysis of the chromatographic conditions of the pesticides is shown in Table 2 and the results of the validation of this analysis in Table 3.

Table 2: Conditions of pesticide analysis.

Gas chromatographer TRACE GC 2000 (Thermo Finnigan)

Column
DB5-MS (J&W, 30 m, 0.25 mm, 0.25 μm)

Program of temperatures
90°C (1 min), 20°C min⁻¹, 180°C (1 min), 2°C min⁻¹, 240°C (1 min), 20°C min⁻¹, 310°C (10 min)

Injector temperature
250°C

Injection volume
1 μL, splitless 0.8 min

Carrier gas
He (N55), 1 mL min⁻¹

Mass spectrometer POLARIS (Thermo Finnigan)

Ionization energy
70 eV

Acquisition mode
Full scan

Mass interval
50–450 amu

Screen speed
1 scan s⁻¹

Acquisition time
32.5 min

Table 3: Results of the methodology validation of the pesticide analysis.

Pesticide
Quantification limit (μg L⁻¹)
Calibration interval (μg L⁻¹)
Validity interval (μg L⁻¹)
Recovery interval (%)

Instrumental step
Full method
Instrumental step
Full method

Isoproturon
20
0.030
20–500
0.030–300
75–130
63–110

Diuron
20
0.030
20–500
0.030–300
82–128
70–123

3,4-Dichloroaniline
20
0.030
20–500
0.030–300
88–130
47–106

4-Isopropylaniline
20
0.030
20–500
0.030–300
80–130
60–125

Desethylatrazine
20
0.030
20–500
0.030–300
76–130
80–129

Trifluralin
20
0.015
20–500
0.030–300
70–130
70–127

Dimethoate
20
0.030
50–500
0.030–300
66–124
54–137

Simazine
50
0.030
20–500
0.030–600
75–135
64–127

Prometon
20
0.030
20–500
0.030–300
76–124
0–125

Atrazine
200
0.100
200–5000
0.100–300
78–130
75–127

Propazine
20
0.015
20–500
0.015–300
86–130
73–127

Terbuthylazine
20
0.015
20–500
0.015–300
79–130
83–128

Parathion methyl
50
0.030
50–500
0.030–300
78–139
72–130

Parathion ethyl
20
0.030
20–500
0.030–300
74–122
64–128

Alachlor
20
0.015
20–500
0.015–300
75–125
70–124

Ametryn
20
0.030
20–500
0.030–300
78–130
0–116

Prometryn
20
0.030
20–500
0.030–300
80–120
17–116

Terbutryn
20
0.030
20–500
0.030–300
80–120
13–114

Chlorpyrifos
20
0.015
20–500
0.015–300
75–120
73–116

Chlorfenvinfos
20
0.015
20–500
0.015–300
76–130
70–126

HCHs
20
0.015
20–500
0.015–300
84–124
70–120

Hexachlorobenzene
20
0.030
20–500
0.030–300
70–130
74–136

Heptachlor
20
0.015
20–500
0.015–300
75–130
58–113

Heptachlor epoxide A
20
0.015
20–500
0.015–300
85–125
62–112

Heptachlor epoxide B
20
0.015
20–500
0.015–300
84–130
58–113

Aldrin
20
0.015
20–500
0.015–300
85–125
64–126

4,4′-Dichlorobenzophenone
20
0.015
20–500
0.015–300
75–120
68–126

Isodrin
20
0.015
20–500
0.015–300
85–125
66–120

α-Endosulphan
20
0.015
20–500
0.015–300
70–125
70–93

pp′-DDE
20
0.015
20–500
0.015–300
89–122
64–107

Dieldrin
20
0.015
20–500
0.015–300
70–125
62–120

Endrin
20
0.015
20–500
0.015–300
80–125
74–122

pp′-DDD + op′-DDT
40
0.030
40–1000
0.030–600
79–125
66–139

Endosulphan-sulphate
20
0.015
20–500
0.015–300
83–125
73–126

pp′-DDT
20
0.030
20–500
0.030–300
76–130
50–120

Dicofol
50
0.030
50–500
0.030–300
80–148
63–136

Methoxychlor
20
0.015
20–500
0.015–300
77–126
75–130

Metholachlor
20
0.015
20–500
0.015–300
76–115
73–128

Molinate
20
0.015
20–500
0.015–300
91–130
75–113

Tetradifon
20
0.015
20–500
0.015–300
85–130
70–116

Previously to the analysis of pesticides by GC/MS a solid-liquid extraction was carried out. This extraction consisted of the retention of organic compounds in a solid phase and subsequent elution with an organic solvent.

The extraction was carried out using an AUTOTRACE WorkStation automatic extractor (Zymark). Before extraction, 100 ng L⁻¹ of surrogate compounds used to control the extraction process (simazine-D5, atrazine-D5, and prometryn-D6) were added to the water sample. During solid-liquid extraction, 900 mL of the sample was passed through cartridges containing a solid ENV+ filter (polystyrene divinyl benzene copolymer) (ISOLUTE cartridges, 200 mg 6 mL⁻¹). The pesticides contained in the sample were retained in the solid phase and dried under for 10 minutes. They were then eluted by passing 10 mL of ethyl acetate (SDS for pesticide analysis) through the cartridge, thus facilitating the passage of these compounds from the water phase to an organic phase. The extracts thus obtained were concentrated under a flow until an approximate volume of 1 mL was obtained, after which 3 mL of isooctane was added (SDS for pesticide analysis) in order to carry out a change of solvent. The extract was then concentrated until obtaining an approximate volume of 0.5 mL. Anthracene deuterate (D10, SUPELCO) was added to each extract as an internal standard for subsequent quantification of the pesticides present in the samples. These extracts were analysed by GC/MS.

2.3. Experimental Procedure in Photocatalytic Treatments

The titanium dioxide used in this work was Degussa P25. The tests were carried out with a dose of 1 g L⁻¹, the same as that used in previous studies and determined as optimal in the stages of disinfection [24–26]. The TiO₂ was added to 1 L of sample to be treated and shaken at 300 rpm to promote contact between the sample and the catalyst and provide oxygen to the reaction medium. The reaction time was 30 minutes.

For tests with hydrogen peroxide, the compound was added to the sample 30% v/v (Carlo Erba) with a concentration of 10 mM. This same dose has been used and determined as optimal in many research works evaluating the effectiveness of photocatalytic treatment with hydrogen peroxide on the degradation of different compounds, or even on the generation of ROS: degradation of dyes [27–30], pesticides [31, 32], antibiotics [33], and ROS production [34]. In these tests, hydrogen peroxide was removed after the treatment.

The UV/VIS radiation source was provided using an ATLAS SUNTEST CPS+/XLS+ solar chamber. This chamber is an instrument equipped with a xenon lamp used for lighting and the ageing of materials. It can be used as simulator of natural sunlight. The chamber also includes an agitation system, a quartz filter, UV radiation, visible light, temperature control. It is equipped with a programmable system for measuring and for regulating the irradiation intensity. The irradiation range of the equipment is from 250 to 785 W m⁻² with a wavelength of 300 to 800 nm. The samples were subjected to a radiation intensity from 500 W m⁻² which corresponds to 50% of the intensity of natural solar radiation at midday [35].

All the tests were performed in duplicate, using glass beakers of 1 L and with shaking. Moreover, blank tests were carried out.

3. Results and Discussion

The results of the two photocatalytic treatments (TiO₂/radiation and /radiation) on the solutions of natural water fortified with the 44 pesticides under study relating to the sample characterization and the removal of pesticides are presented below.

3.1. Sample Characterization

The characterization of the initial sample and the samples treated with the two photocatalytic processes is shown in Table 4.

Table 4: Characterization of samples of surface wáter fortified with pesticides in photocatalytic treatments.

Initial sample

Physical-chemical parameters

pH
8.0
Colour (PCU)
0

Conductivity (μS cm⁻¹)
508
Dissolved oxygen (mg L⁻¹)
9.9

Turbidity (NTU)
69
Hardness (mg CaCO₃ L⁻¹)
108

Suspended solids (mg L⁻¹)
27
concentration (mg L⁻¹)
0.15

Phosphorous (mg L⁻¹)
0.8
CN⁻ concentration (mg L⁻¹)
0.002

DOC (mg C L⁻¹)
22
Toxicity (% inhibition)
33.6

Chlorine demand (mg Cl₂ L⁻¹)
6.0

Anion concentrations in solution (mg L⁻¹)

Fluorides
0.1
Nitrates
10.8

Chlorides
63.8
Phosphates
<0.2

Sulphates
98

Sample after treatment TiO₂/radiation

Physical-chemical parameters

pH
8.0
Colour (PCU)
0

Conductivity (μS cm⁻¹)
589
Dissolved oxygen (mg L⁻¹)
6.2

Turbidity (NTU)
54
Hardness (mg CaCO₃ L⁻¹)
109

Suspended solids (mg L⁻¹)
7
concentration (mg L⁻¹)
0.15

Phosphorous (mg L⁻¹)
0.4
CN⁻ concentration (mg L⁻¹)
0.0015

DOC (mg C L⁻¹)
14
Toxicity (% inhibition)
18.9

H₂O₂ concentration (mg L⁻¹)
5
Chlorine demand (mg Cl₂ L⁻¹)
1.0

Anion concentrations in solution (mg L⁻¹)

Fluorides
<0.1
Nitrates
11.6

Chlorides
66.4
Phosphates
<0.2

Sulphates
101.0

Sample after treatment TiO₂/H₂O₂/radiation

Physical-chemical parameters

pH
8.0
Colour (PCU)
0

Conductivity (μS cm⁻¹)
625
Dissolved oxygen (mg L⁻¹)
9.8

Turbidity (NTU)
56
Hardness (mg CaCO₃ L⁻¹)
95

Suspended solids (mg L⁻¹)
5
concentration (mg L⁻¹)
0.15

Phosphorous (mg L⁻¹)
0.2
CN⁻ concentration (mg L⁻¹)
0.001

DOC (mg C L⁻¹)
22
Toxicity (% inhibition)
43.3

H₂O₂ concentration (mg L⁻¹)
0.5
Chlorine demand (mg Cl₂ L⁻¹)
55.6

Anion concentrations in solution (mg L⁻¹)

Fluorides
<0.1
Nitrates
11.4

Chlorides
65.0
Phosphates
<0.2

Sulphates
99.2

The results show the following. (i)The application of photocatalytic treatments does not produce significant variations with respect to the initial values of pH, conductivity, color, dissolved oxygen, hardness, ammonium concentration, and concentration of ions in solution. (ii)The turbidity of the sample, in both cases, declines slightly, and solids in suspension make it significantly through the chemical oxidation of organic matter produced in the treatment. (iii)Chlorine demand in the TiO₂/radiation-treated sample is reduced significantly, up to a value of of 1 mg L⁻¹. However, the application of TiO₂/H₂O₂/radiation treatment produces an enormous increase in this parameter. This is possibly due to the various byproducts formed after the application. (iv)Regarding the toxicity of treated sample, TiO₂/radiation treatment reduces the initial toxicity of the sample to approximately half its initial value. On the other hand, the application of TiO₂/H₂O₂/radiation produces an increase in the initial toxicity, due to the formation of byproducts more toxic than the original. This has occurred in other research works concerning the application of these photocatalytic treatments [14, 36–38]. (v)The COD undergoes no variation through the TiO₂/H₂O₂/radiation treatment while it is reduced by 36% after the application of TiO₂/radiation. This suggests the mineralization of organic matter present in the process with hydrogen peroxide [39]. (vi)Suspended solids are removed by both photocatalytic treatments. This is due to the organic matter present in the water by this advanced oxidation process. (vii)Hydrogen peroxide appears after the TiO₂/radiation treatment formed as a by-product in photocatalytic reactions. In the case of the TiO₂/H₂O₂/radiation treatment, where it is used initially as a reagent, almost its total consumption is observed.

3.2. Pesticide Removal

Photocatalytic experiments were carried out in duplicate. Table 5 shows the average concentration of each pesticide studied in the initial sample and the final concentration after the photocatalytic treatments. It is worth noting that average concentrations are shown because the variations obtained in these analyses were very low, always below 2%.

Table 5: Concentration of pesticides in samples in photocatalytic treatments.

Pesticide
Concentration (ng L⁻¹)

Initial
After TiO₂/radiation
After TiO₂/H₂O₂/radiation

Alachlor
505
253
177

Aldrin
512
230
26

Ametryn
501
225
225

Atrazine
551
358
331

Chlorfenvinfos
492
295
221

Chlorpyrifos
520
104
104

pp′-DDD
510
332
102

op′-DDE
480
288
144

op′-DDT
482
313
96

pp′-DDT
482
386
145

Desethylatrazine
593
385
356

3,4-Dichloroaniline
658
0
0

4,4′-Dichlorobenzophenone
519
493
363

Dicofol
568
57
57

Dieldrin
508
356
356

Dimethoate
608
274
243

Diuron
501
125
125

α-Endosulphan
475
48
0

Endosulphan-sulphate
483
459
435

Endrin
486
243
243

α-HCH
511
109
409

β-HCH
519
441
441

γ-HCH
521
443
417

δ-HCH
504
428
428

Heptachlor
491
246
246

Heptachlor epoxide A
495
347
347

Heptachlor epoxide B
487
341
341

Hexachlorobenzene
503
327
327

Isodrin
516
206
0

4-Isopropylaniline
512
0
0

Isoproturon
521
78
78

Metholachlor
524
262
210

Methoxychlor
519
234
130

Molinate
551
248
165

Parathion ethyl
507
228
228

Parathion methyl
508
51
51

Prometon
492
271
271

Prometryn
489
220
220

Propazine
508
330
305

Simazine
554
305
277

Terbuthylazine
524
262
262

Terbutryn
514
231
206

Tetradifon
493
296
296

Trifluralin
566
255
170

In addition to the photocatalytic treatments, blank tests were carried out. These blank tests were solution of pesticides with titanium dioxide without radiation and with radiation without catalyst. The results obtained are shown in Table 6. Only for some pesticides were degradations different to zero achieved.

Table 6: Degradation of pesticides in blank experiments.

Pesticide
Removal by TiO₂
Removal by radiation

Trifluralin
0
28

Heptachlor
14
45

Aldrin
20
50

Isodrin
24
55

pp′-DDE
33
46

pp′-DDD + op′-DDT
21
13

pp′-DDT
25
28

As can be observed, some of the studied pesticides can be degraded by TiO₂ without radiation and by radiation without TiO₂. The degradation of these pesticides by TiO₂ only is due to their adsorption in the catalyst. The degradation of these pesticides by radiation only is due to the advanced oxidation process with UV by which hydroxyl radicals are generated.

Table 7 shows the average removal percentages of the pesticides.

Table 7: % Removal of pesticides by photocatalytic treatments.

Group
Pesticide
Removal after TiO₂/radiation (%)
Removal after TiO₂/H₂O₂/radiation (%)

Triazines
Simazine
45
50

Atrazine
35
40

Propazine
35
40

Terbuthylazine
50
50

Prometon
45
45

Ametryn
55
55

Prometryn
55
55

Terbutryn
55
60

Desethylatrazine
35
40

Organic phosphorated
Parathion methyl
90
90

Parathion ethyl
55
55

Chlorpyrifos
80
80

Chlorfenvinfos
40
55

Dimethoate
55
60

HCHs
α-HCH
20
20

β-HCH
15
15

χ-HCH
15
20

δ-HCH
15
15

HCB
Hexachlorobenzene
35
35

Heptachlors
Heptachlor
50
50

Heptachlor epoxide A
30
30

Heptachlor epoxide B
30
30

Endosulphans
α-Endosulphan
90
100

Endosulphan-sulphate
5
10

Drins
Endrin
50
50

Dieldrin
30
30

Isodrin
60
100

Aldrin
55
95

DDTs
pp′-DDE
40
70

pp′-DDD + op′-DDT
35
80

pp′-DDT
20
70

Anilines
3,4-Dichloroaniline
100
100

4-Isopropylaniline
100
100

Ureas
Isoproturon
85
85

Diuron
75
75

Carbamate
Molinate
55
70

Nitroderivate
Trifluralin
55
70

Anilides
Alachlor
50
65

Metholachlor
50
60

Methoxychlor
55
75

Chlorophenols
Tetradiphon
40
40

Dicofol
90
90

Chlorinated diphenyl
4,4′-Dichlrobenzophenone
5
30

Average
48
57

The photocatalytic treatments achieved an average degradation of 48% by photocatalysis (TiO₂/radiation) and 57% by photocatalysis with hydrogen peroxide (TiO₂/H₂O₂/radiation).

The photocatalytic treatments were very effective in the removal of parathion methyl, chlorpyrifos, α-endosulphan, 3,4-dichloroaniline, 4-isopropylaniline, and dicofol. The treatments were less effective in the degradation of HCHs, endosulphan-sulphate, heptachlors epoxide, and 4,4′-dichlorobenzophenone.

As can be seen in Tables 5 and 7, the addition of hydrogen peroxide to the process slightly improves the average percentage of pesticide removal. However, this improvement does not occur for all the pesticides studied. In fact, some of them present the same removal percentages by both photocatalytic treatments. The increase in the average percentage of degradation is due to an improvement in some of the studied pesticides to add hydrogen peroxide. These are some triazines, α-endosulphan, molinate, trifluralin, and anilides, for which removal is improved by 5–20% using hydrogen peroxide; isodrin, aldrin, DDTs, and 4,4′-dichlorobenzophenone, for which removal is improved by 25–50% using hydrogen peroxide.

The degradation percentages of the pesticides in their individual group are as follows.

TriazinesThe rates of degradation of triazines obtained by the photocatalytic treatments are between 35 and 60%. By photocatalysis, these percentages are from 35 to 55%. Degradation of these pesticides is very similar, 40–60%, when hydrogen peroxide is added to the treatment. Therefore, the addition of this reagent is not compensated for by the increase of the effectiveness in the degradation of triazines by photocatalytic treatment. Among the triazines studied are atrazine and desethylatrazine, the most difficult to degrade by photocatalytic treatments.

Organic PhosphoratedThe five organic-phosphorated pesticides studied are degraded between 40 and 90% by photocatalytic treatments. The degradation percentages obtained for these pesticides are similar whether or not hydrogen peroxide is added. The degradation of dimethoate and chlorfenvinfos only slightly improves when hydrogen peroxide is added. Methyl parathion and chlorpyrifos are the pesticides that degrade best by photocatalysis (degradation of 80–90%) while chlorfenvinfos is the most difficult organic-phosphorated pesticide to degrade (maximum degradation of 55%).

HCHs and HCBWith regard to HCHs, the degradation percentages obtained by the photocatalytic treatments are the lowest. In all cases, adding hydrogen peroxide makes no difference to the degradation percentages obtained so its use is redundant. The HCB is degraded 35% by the photocatalyisis, and the addition of hydrogen peroxide does not produce any improvement.

Heptachlors Degradation rates obtained for the heptachlors are 50% while the heptachlors epoxides are more difficult to degrade (30%). The addition of hydrogen peroxide to the process does not improve the degradation percentages of these pesticides. In case of heptachlors, the blanks experiments show a degradation of 45% by radiation. The photocatalytic treatment does not therefore improve its degradation with respect to radiation only.

EndosulphansUnder photocatalysis, α-endosulphan shows almost total degradation while endosulphan-sulphate is practically undegraded by photocatalytic treatments. Both have the same behavior after the addition of hydrogen peroxide, showing an improvement of 5%, although again this small improvement does not compensate for its use.

DrinsWith regard to these pesticides, it can be seen by these treatments that aldrin and isodrin (isomers between them) are more easily degradable that endrin and dieldrin (isomers between them), especially in the case of using hydrogen peroxide. The rates of degradation of dieldrin and endrin, 50% and 30% respectively, do not improve when adding hydrogen peroxide. In the case of aldrin and isodrin, blank experiments show an important degradation by radiation. Thus radiation and not the photocatalytic treatment is responsible for their degradation. However, the degradation of aldrin and isodrin is noticeably more effective when using hydrogen peroxide treatment, achieving their complete degradation.

DDTs The DDTs studied degrade between 20% and 40% under the photocatalytic treatments. However, similar degradations are achieved in the blank experiments. The degradation could thus occur by the effect of titanium dioxide or radiation only, not by photocatalytic treatment. These degradation rates increase significantly by adding hydrogen peroxide, reaching 70–80%. Therefore, for the DDTs the presence of hydrogen peroxide notably favours their degradation.

AnilinesThe two studied anilines, 3,4-dichloroaniline and 4-isopropylaniline, are completely degraded by the photocatalytic treatments. Therefore, the use of hydrogen peroxide is not necessary for increasing the effectiveness of the treatment.

UreasIsoproturon and diuron, the studied pesticides that are derivatives of urea, present high degradation rates under the photocatalytic treatments, being 85% and 75%, respectively. The same percentages are obtained when using hydrogen peroxide, so that its use does not improve the process.

AnilidesThe three anilides studied also show the same behaviour with photocatalytic treatments, and the addition of hydrogen peroxide produces a slight improvement, from 5% to 20%. Under photocatalysis, the degradation percentages are 50–55%. These percentages increase slightly when hydrogen peroxide is added to the treatment system, achieving rates of 60–75% degradation in this case.

Other PesticidesFor the rest of the pesticides under study, molinate, trifluralin, tetradiphon, dicofol, and 4,4′-dichlorobenzophenone, very different degradations are achieved. Molinate and trifluralin are degraded 55% by the photocatalysis and this percentage increases to 70% when hydrogen peroxide is added. Therefore, the use of this reagent represents a significant improvement in the degradation of these two pesticides. In the case of trifluralin, the blank experiment with radiation produced 30% degradation but its removal is greater by photocatalysis. Degradation of dicofol is very effective by the photocatalytic treatment, being 90%. The presence of hydrogen peroxide does not favour degradation in this case. The same applies to tetradiphon, which degrades to a lesser extent (40%), but hydrogen peroxide does not increase the effectiveness of the process. The 4,4′-dichlorobenzophenone is barely degraded at all by photocatalysis but its degradation increases up to 30% when adding hydrogen peroxide to the treatment.

4. Conclusions

After the completion of this study, the following conclusions can be drawn.

(i)The pesticides most effectively removed by the photocatalytic treatments, reaching yields higher than 80%, are parathion methyl, α-endosulphan, chlorpyrifos, 3,4-dichloroaniline, 4-isopropylaniline, and dicofol. The least degraded, below 30%, are HCHs, endosulphan-sulphate, heptachlors epoxides, and 4,4′-dichlorobenzophenone.(ii)Photocatalytic treatment, TiO₂/radiation, achieves a partial removal of the studied pesticides of 48%. When hydrogen peroxide is added the average percentage increases to 57%. The addition of hydrogen peroxide improves the degradation of some of the pesticides studied, mainly isodrin, aldrin, DDTs, 4,4′-dichlorobenzophenone, some triazines, molinate, α-endosulphan, trifluralin, and anilides.(iii)TiO₂/radiation treatment produces a reduction of 36% of the initial COD, 43% of the initial toxicity, and chlorine demand is reduced to 1 mg L⁻¹. The same treatment with hydrogen peroxide produces a large increase in toxicity and chlorine demand of the treated sample and there is no variation in the COD due to the generation of intermediate compounds (no mineralization) that can be more toxic than the original.(iv)Both the studied photocatalytic processes produce a reduction of turbidity and of the solids in suspension in the treated samples.

Acknowledgments

The authors wish to thank “MICINN-FEDER” for financing this paper through the project “Regeneración de aguas depuradas mediante procesos de oxidación avanzada (CTM2008-01876/TECNO).”

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