Degradation of Gesaprim Herbicide by Heterogeneous Photocatalysis Using Fe-Doped TiO2

doi:10.4236/ijg.2011.24068

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International Journal of Geosciences, 2011, 2, 669-675

doi:10.4236/ijg.2011.24068 Published Online November 2011 (http://www.SciRP.org/journal/ijg)

669

Degradation of Gesaprim Herbicide by Heterogeneous

Photocatalysis Using Fe-Doped TiO2

Noemí Acevedo Quiroz1, Dulce Jocelyn Ramos Gutierrez1, Susana Silva Martínez2,

Cristina Lizama Bahena1

1Posgrado de la Facultad de Ciencias Químicas e Ingeniería y Centro de Investigación en Ingeniería y Ciencias

Aplicadas, Universid ad Autónoma del Estado de Morelos, Cuernavaca, Mexico

2Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos,

Cuernavaca, Mexico

E-mail: [email protected]

Received June 21, 2011; revised August 7, 2011; accepted September 26, 2011

Abstract

Fe-doped TiO2 was prepared by the sol gel method and characterized by X-ray diffraction. All the Fe-doped

TiO2 were composed of an anatase crystal form. The activity of the Fe-doped TiO2 for the degradation of the

gesaprim commercial herbicide (which contains atrazine as active compound and formulating agents) was

studied by varying the iron content during UV (15 W), visible light and solar irradiations. The visible light

came from commercial saving energy lamps (13, 15 and 20 Watts). The gesaprim degradation rate depended

on the iron content in the photo catalyst. The Fe-doped TiO2 (0.5% by weight of TiO2) showed higher TOC

removal under visible light and was more active than the undoped TiO2 photo catalyst under the light irradia-

tion sources tested. Over 90% of chemical oxygen demand abatement was achieved with both UV and visi-

ble light but less time was required to decrease the chemical oxygen demand content by using the catalyst

doped with iron at 0.5% under visible light. It was observed that the degradation of gesaprim increased by

increasing the iron content in the catalyst under visible light.

Keywords: Atrazine, Iron Doped TiO2, Solar Irradiation, Visible Light Irradiation

1. Introduction

Nowadays, the use of herbicides in agriculture activities to

control weedy plants and to increase food production has

become an important tool to the detriment of the en- vi-

ronment. The herbicides and their degradation products

(complex metabolites) may alter the natural habitats for

different plant and animal species depending on how they

are transported in the environment. Chlorinated tri- azines

are herbicides widely used for selective weed control, and

are amongst the most commonly used herbicides in the

world [1]. The main compound of this family is the atrazine

(2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and it

is found in the environment contaminating soil and water

reserves [2]. This herbicide belongs to the persistent or-

ganic pollutants because of its low biodegradability and

long half-life (60 and 100 days) in water.

Several processes have been developed to remove

atrazine from aqueous wastes, such as activated carbon

and its combination with ozone [3,4], adsorption onto

carbon nanotubes [5], adsorption onto zeolites [6], ad-

sorption on fractionated Al-pillared and Fe-Al-pillared

clays [7], photochemical degradation in the presence of

hydrogen peroxide and microwave [8], reverse osmosis

[9], ozone [10], hydrogen peroxide with ozone [11], elec-

tro-Fenton [12], Fenton and photo-Fenton [13], photoly-

sis by TiO2 semiconductor [14-18], atrazine by nanoscale

zero valent iron supported on organobentonite [19], and

hydrogen peroxide with TiO2, and Fe2O3 semiconductor

suspensions assisted by light [20-22], among others.

Amongst the treatment methods, the semiconductor

photocatalytic process for the photocatalytic oxidation

mediated by titanium dioxide appears to be an effective

strategy for degrading chlorinated triazine herbicides

because the semiconductor has shown a great potential as

a low-cost, and environmental friendly alternative for

wastewater treatment. Besides, this advanced oxidation

process has been widely demonstrated to remove persis-

tent organic compounds and microorganisms in water.

The photocatalytic oxidation of s-triazine herbicides and

670 N. A. QUIROZ ET AL.

the pathways of their degradation intermediates have

been reported by several studies [22,23]. It has been con-

cluded that the oxidation of the lateral chains and subse-

quent disappearance of the initial compound is very fast,

but the formation of the final product (cyanuric acid)

may require a long irradiation time [23]. Also, it has

been reported that complete mineralization of atrazine

could not be attained because of the stability of the s-

triazine ring toward oxidation which only affects the

lateral chains with 5 of the 8 carbons removed as CO2.

Hustert et al. studied the photocatalytic treatment of

atrazine herbicide, and also other s-triazine herbicides

(simazine, and cyanzine) [19]. These researchers re-

ported that the degradation of the s-triazines occurred in

various steps with cyanuric acid was formed as end

product of degradation. Hequet et al. studied the photo-

lytic degradation of atrazine and found that the UV

photolysis was efficient [24]. They reported that the main

degradation pathway was deshalogenation and hydroxya-

trazine was generated as the main intermediate with

cyanuric acid as the final end product. These authors also

studied the photocatalytic degradation of atrazine, and

found that the major intermediates were desalkylated

compounds with cyanuric acid as the final degradation

product. McMurray and coworkers reported that the pri-

mary pathway for atrazine degradation on nanoparticu-

late TiO2 films involved the oxidation of the lateral side

chains of atrazine, producing dealkylated derivatives (2-

chloro-4-acctamido-6-isopropylamino-1,3,5-triazine, 2-

chloro-4-ethylamino-6-(2-propanol)amino-1,3,5-triazine,

2-chlor o-4-ethyla mino-6 -(2-propanol)amino-1,3,5-triazin

e, desethylatrazine, deisopropylatrazine and desethylde-

sisopropyl atrazine) [16]. They also reported that the

hydrolysis of the chlorine substituent in desethylatrazine,

desethyldesisopropyl atrazine and deisopropylatrazine

led to the production of 2-hydroxydesethyl atrazine with

the further displacement of the amino groups by hy-

droxyl groups resulting in cyanuric acid. These authors

proposed a secondary pathway which follows a hy-

droxylated pathway with immediate substitution of the

chlorine at position two with a hydroxyl group. They

neither observed complete mineralization of atrazine nor

photolysis of atrazine either with the UVA or UVB

source. Peñuela and Barceló studied the photo degrada-

tion of atrazine and desethylatrazine in water with TiO2/

H2O2 and FeCl3/H2O2 by using a xenon arc lamp and

sunlight irradiation [25]. These authors reported that

atrazine degraded faster than desethylatrazine in the

presence of FeCl3 using both light sources with half-lives

varying from 5 to 11 min and from 19 to 26 min, respec-

tively. Měšt’ánková et al., reported that the degradation

rate of the herbicide monuron was enhanced when TiO2

was in contact with Fe(III) (from Fe(ClO4)3) because the

Fe(III) acted as scavenger of electrons photogenerated in

TiO2 particles which let down hole–electron recombina-

tion [26].

The doping with small amounts of metal impurities

aims at facing one relevant drawback of TiO2, which is

its poor absorption of visible light. Enhanced visible light

absorption has been reported in the presence of transition

metals [27,28], nitrogen and carbon as dopants [29]. It

was reported that the UV-Vis absorption studies showed

significantly enhanced red-shift in UV-Vis absorption

spectra with an increased amount of iron [30,31]. The

photocatalytic activity of nano-sized Fe-doped TiO2,

examined by the mineralization of oxalic acid under

visible light irradiation, showed highest activity with 2%

of Fe-doped TiO2 [31]. However, it was reported that the

photocatalytic efficiency of metal doped TiO2 strongly

depends on the conditions of preparation, on the mor-

phostructural properties (crystalline structure, specific

surface area) and on the type of the organic pollutant to

be photooxidized.

The aim of this work was to synthesize TiO2 and iron-

doped TiO2 with the sol-gel method and to study the ef-

ficiency of Fe-doped titania suspensions in the photo-

catalytic degradation of the gesaprim commercial herbi-

cide under UV light, visible light and solar light. Ge-

saprim, herbicide used for the control of broadleaf weeds

and some grassy weeds, contains atrazine as active com-

pound and formulating agents as additional components.

2. Material and Methods

2.1. Chemicals

Gesaprim herbicide (90 GDA) was directly purchased

from Syngenta Crop Protection Inc. (USA); the gesaprim

contained 90% of atrazine as active ingredient. Titanium

tetrabutoxide (Ti(OC4H9)4), anhydride ethanol, distilled

water, ferric chloride (FeCl3), HNO3 and H2SO4 were

analytical grade (Sigma-Aldrich). All chemicals were

used as received without further purification.

2.2. Synthesis of Fe-Modified TiO2 Photo

Catalyst

The Fe-doped TiO2 catalyst was prepared by the sol–gel

method as below: A mixture of H2O (1.25 ml), HCl (0.5

ml), C2H6O (5 ml) and FeCl3 (different iron amounts)

was prepared with stirring for 5 min. Separately,

Ti(OC4H9)4 solution at a concentration of 44.7 g/l ex-

pressed as TiO2 was added drop wise to 22.5 ml C2H6O

solution (99.9%) with stirring under nitrogen atmosphere,

this solution was kept under stirring during 20 min. Then,

the iron mixture was added drop by drop to the ethanol-

671

N. A. QUIROZ ET AL.

titanium tetrabutoxide solution and kept under stirring

during 2 h until a gel was formed. The content of iron

was 0.0%, 0.1% and 0.5% by weight of TiO2. Afterwards,

the solvent was eliminated under vacuum. The resulting

powder was dried at 100˚C for 30 min followed by ther-

mal treatment at 400˚C with air during 2 h. The crystal

structure of the powders was studied by X-ray diffraction

on a diffractometer (Rigaku model DMAX 2200) with

Cu Ka radiation (



= 1.54439 Å) at 2



= 0˚ - 80˚, and the

morphology by the scanning microscope electron (LEO

1450 VP).

2.3. Photocatalytic Degradation of Gesaprim

Herbicide

The photocatalytic degradation of gesaprim was carried

out employing white fluorescent light (visible light from

an energy saving compact lamp), UV and solar irradia-

tion. Gesaprim degradation, using TiO2 and Fe-doped

TiO2 suspension of the powders synthesized, was fol-

lowed by measuring the chemical oxygen demand (COD)

and Total organic carbon (TOC). The initial concentra-

tion of gesaprim which corresponds to its maxima solu-

bility in water at pH 3 is 40 ppm. Such concentration has

a COD and TOC content of 38 ppm and 20 ppm, respec-

tively. The photocatalytic activity of the photo catalyst

Fe-doped TiO2 was evaluated by measuring the decom-

position rate of gesaprim in a circulation type reactor.

The reactor was similar to a laboratory glassware con-

denser (made of glass Duran, 20 cm long and 4 cm

inner diameter). The UV or visible lamp was put inside

the inner glass tube of the condenser so that the irradia-

tion of the UV/Vis light was ~2 mm away from the

flowing gesaprim solution; while the gesaprim solution

was running in a similar way that a coolant does in a

laboratory glassware condenser using a centrifugal poly-

propylene circulating pump (Cole Parmer). Stock solu-

tions of gesaprim (0.106 g diluted in 2.5 L) were pre-

pared with distilled water at pH 3 adjusted with H2SO4

with gentle stirring during 2 d. The TiO2 (iron doped/

undoped) slurry solution, containing 40 ppm of initial

concentration of gesaprim at pH 3, in the reactor was

irradiated with a UV light (15 W, 352 nm, Cole Parmer)

or energy saving white fluorescent light (13, 15, and 20

W compact lamps, Phillips). The UV illumination inten-

sity at the surface of the sample was 0.068 W/cm2;

whereas it varied inside of 0.059 - 0.090 W/cm2 range

for the visible light. The gesaprim solution was in con-

tact with the photo catalyst for 20 min previous to switch

the lamp on. The COD of gesaprim in the reactor were

measured as a function of the photocatalytic degradation

time which started at the moment the lamp was on;

whereas TOC was analyzed when the experiment was

terminated. COD and TOC were analyzed using standard

methods and standard tubes (Hach) inside the concentra-

tion range of 0 - 40 ppm. Samples were filtered as col-

lected prior the analysis.

3. Results and Discussion

3.1. Characterization of Undoped TiO2 and Iron

Doped TiO2 Powders

Figure 1 shows the XRD patterns of the TiO2 and 0.5%

Fe doped TiO2 powder annealed at 400˚C. According to

the main reflection, the observed peaks can be attributed

to the anatase phase. Regarding to the Fe-doped TiO2,

the diffraction peaks for iron are completely missing

from the XRD pattern for the doped iron TiO2 powder.

This may be attributed to the low sensitivity of the XRD

method due to the low concentration of added iron [32].

It could also be attributed to the incorporation of iron

ions in the crystal structure of TiO2 [33], or by very fine

dispersion of iron in the titania resulting in X-ray amor-

phous behavior [32]. Ambrus and coworkers doped the

titania with iron up to 10% of iron content (Fe(III)) and the

diffraction peaks for iron were completely missing from

the XRD pattern for the doped iron TiO2 powder [32].

3.2. Influence of Iron on the Photo Catalyst for

the Degradation of Gesaprim under Visible

Light

The photocatalytic activity of the Fe-doped TiO2 was

observed for the decomposition of gesaprim which was

carried out employing white fluorescent light (visible

light) and solar irradiation. The COD decay profile for

the decomposition of gesaprim (Figure 2(a)) increased

by the presence of iron in the TiO2 photo catalyst (400

mg/L) under visible light irradiation (with 0.090 W/cm2

using 20 W lamp). The highest initial degradation rate of

Figure 1. XRD patterns of TiO2 and iron-doped TiO2

powder.

672 N. A. QUIROZ ET AL.

gesaprim was found to be at 0.5% of iron content (by

weight of TiO2) achieving 95% of COD removal after 60

min of visible light irradiation (with an energy consump-

tion of 288 kJ/L) which corresponds to a TOC removal

of ~50% at the same time. Increasing the degradation

time further, it was observed that TOC was decreased

90% of its original value (20 ppm) at 200 min of photo

reaction.

While the undoped TiO2 photo catalyst showed ~21%

of COD removal at the same time under the same condi-

tions. The degradation of gesaprim under solar irradia-

tion (in a cloudless day from 10:00 to 14:00 h) is shown

in Figure 2(b). It can be seen from this figure that the

initial COD decay (decomposition rate) of gesaprim is

fast in the first 10 minutes and then the decomposition of

gesaprim slows down. COD abatements of 13%, 36%

and 65% were, respectively, achieved by undoped TiO2,

0.1% Fe-doped TiO2 and 0.5% Fe-doped TiO2 after 60

minutes under solar irradiation at pH 3 (with an energy

consumption of 267.9 kJ/L). TOC removal of ~23% was

achieved using 0.5% Fe-doped TiO2 under solar light at

60 min of reaction. These results show that the increased

activity of TiO2 doped with iron is because of the nature

of iron which can be an electron or hole scavenger and

results in the improvement of the separation of free car-

(a)

(b)

Figure 2. COD profile for the degradation of gesaprim

using TiO2 and iron-doped TiO2 (catalyst load: 400 ppm

and pH 3) under (a) visible light (20 W) and (b) solar

irradiat ion.

riers. Thus, the recombination rate of the electron–hole

pairs is lowered and the photocatalytic activity is en-

hanced. It has been reported that the photocatalytic activ-

ity of iron doped TiO2 strongly depends on the prepara-

tion method, iron precursor, and the amount and state of

iron (commonly a very low content of doped iron has a

positive effect on the enhancement of the photocatalytic

activity of Fe-doped TiO2).

It has also been reported that higher amount of Fe(III)-

doped TiO2 than an optimal iron content is detrimental

on the photocatalytic activity of the photo catalyst be-

cause the Fe(III) at high concentration in Fe-doped TiO2

can act as a recombination centres [34].

3.3. Influence of Light Irradiation on the Photo

Catalyst for the Degradation of Gesaprim

In order to study the effect of light source on the degra-

dation of gesaprim by 0.5% Fe-doped TiO2 catalyst, sev-

eral experiments were carried out and recorded as a func-

tion of the energy consumption to compare the degrada-

tion efficiency of gesaprim for each light source. It was

used different power lamps (Figure 3) and different

source of light irradiation (Figure 4) to record the COD

abatement profiles during the degradation of gesaprim.

Figure 3 reports that the initial rate of COD abatement

is fast and similar in the first 100 minutes of degradation

regardless of the power of the lamp used. Afterwards, the

COD was 95% abated when the lamp of the higher

power (20 W) was used in which it was required 300

kJ/L of energy consumption (after 60 min of photo ca-

talysis), attributable mainly to visible-light absorption by

the iron compound; whereas, only 85% of COD was

abated with the other two lamps even at higher energy

consumption.

The profiles for COD abatement using different sources

Figure 3. COD removal profile for the degradation of ge-

saprim using 400 ppm of 0.5% Fe-doped TiO2 under dif-

ferent powers of the visible light.

673

N. A. QUIROZ ET AL.

(a)

(b)

Figure 4. Photocatalytic degradation of gesaprim using 400

ppm of 0.5% Fe-doped TiO2 under different sources of light

irradiation.

of light irradiation (Figure 4) clearly show an improv-

ment in the degradation of gesaprim by the presence of

iron in the catalyst (0.5% Fe-doped TiO2) under visible

light from both the 20W Vis-lamp and the solar irradia-

tion (Figure 4(a)).

It is interesting to observe that the iron-doped TiO2

enhanced the initial rate of COD removal under the solar

irradiation with respect to that obtained with or without

undoped TiO2 under similar energy efficiency. Thus, the

iron in the catalyst increased the reactivity of the photo

catalyst under the visible-light of the solar irradiation.

COD removals of 23%, ~40% and 74% were achieved

by the sole effect of solar photolysis, solar photo cataly-

sis and solar Fe-doped photo catalysis, respectively. The

activation of the catalyst under solar irradiation is by

both visible and UV irradiation (with ~6% of UV light

contribution [35]. This figure also shows that 95% of

COD removal was obtained by the Fe-doped TiO2 under

visible irradiation (20 W lamp) after 60 minutes of reac-

tion (with an energy consumption of 288 kJ/L). The high

COD removals achieved is because the iron deposition

on TiO2 surface improves the photocatalytic activity by

suppressing the electron-hole recombination [36].

In our previous study [14], it was shown that the

photodegradation of this commercial herbicide was en-

hanced by the use of ultrasound in the presence of TiO2

catalyst with COD removal of 84% at 150 minutes of

UV light irradiation. The combination of these two proc-

esses (sonophotocatalysis) gave very high decomposition

yields of the active compound (atrazine) reaching high

degree of mineralization (97%) [14]. Thus, a comparison

of the results reported here with those obtained in our

previous study using the sonophotocatalytic process, it is

interesting to note that the employment of iron-doped

TiO2 catalyst (under visible light) gives similar results at

shorter time.

Figure 4(b) reports similar COD removal profiles un-

der the UV lamp irradiation (15W) with the achievement

of higher COD removals in comparison with those

reached under solar irradiation (with comparable energy

consumption in both systems). From these results, it

might be assumed that Fe-modification also improves

inherent photocatalytic activity of the TiO2 powders to

some extent because of an efficient charge separation of

the UV light generated electron-hole pairs.

4. Conclusions

The gesaprim degradation rate depended on the iron con-

tent in the photo catalyst. The Fe-doped TiO2 (0.5% by

weight of TiO2) showed higher TOC removal under visi-

ble light and was more active than the undoped TiO2

photo catalyst under the light irradiation sources tested.

Over 90% of chemical oxygen demand abatement was

achieved with both UV and visible light but less time

was required to decrease the chemical oxygen demand

content by using the catalyst doped with iron at 0.5%

under visible light (90% of TOC was removed after 200

min of photoreaction). It was observed that the degrada-

tion of gesaprim increased by increasing the iron content

in the catalyst under visible light. This can be explained

by the fact that the Fe(III) acted as scavenger of electrons

photogenerated in TiO2 particles which let down hole–

electron recombination.

This advanced oxidation process offers an environ-

mental alternative for the treatment of water polluted

with this commercial herbicide.

5. Acknowledgements

This work was financially supported by the Programa de

Mejoramiento del Profesorado (PROMEP) of the Secre-

taría de Educación Pública (SEP). We thank CONACyT

for the Grant given to N.A.Q and D.J.R.G. to support

their postgraduate studies.

674 N. A. QUIROZ ET AL.

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