Degradation of Gesaprim Herbicide by Heterogeneous Photocatalysis Using Fe-Doped TiO 2

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 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. It was observed that the degradation of gesaprim increased by increasing the iron content in the catalyst under visible light.


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-vironment.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 organic pollutants because of its low biodegradability and long half-life (60 and 100 days) in water.
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 persistent organic compounds and microorganisms in water.The photocatalytic oxidation of s-triazine herbicides and the pathways of their degradation intermediates have been reported by several studies [22,23].It has been concluded that the oxidation of the lateral chains and subsequent 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 striazine ring toward oxidation which only affects the lateral chains with 5 of the 8 carbons removed as CO 2 .Hustert et al. studied the photocatalytic treatment of atrazine herbicide, and also other s-triazine herbicides (simazine, and cyanzine) [19].These researchers reported 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 photolytic degradation of atrazine and found that the UV photolysis was efficient [24].They reported that the main degradation pathway was deshalogenation and hydroxyatrazine 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 primary pathway for atrazine degradation on nanoparticulate TiO 2 films involved the oxidation of the lateral side chains of atrazine, producing dealkylated derivatives (2chloro-4-acctamido-6-isopropylamino-1,3,5-triazine, 2chloro-4-ethylamino-6-(2-propanol)amino-1,3,5-triazine, 2-chloro-4-ethylamino-6-(2-propanol)amino-1,3,5-triazin e, desethylatrazine, deisopropylatrazine and desethyldesisopropyl 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 hydroxyl groups resulting in cyanuric acid.These authors proposed a secondary pathway which follows a hydroxylated 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 degradation of atrazine and desethylatrazine in water with TiO 2 / H 2 O 2 and FeCl 3 /H 2 O 2 by using a xenon arc lamp and sunlight irradiation [25].These authors reported that atrazine degraded faster than desethylatrazine in the presence of FeCl 3 using both light sources with half-lives varying from 5 to 11 min and from 19 to 26 min, respectively.Měšt'ánková et al., reported that the degradation rate of the herbicide monuron was enhanced when TiO 2 was in contact with Fe(III) (from Fe(ClO 4 ) 3 ) because the Fe(III) acted as scavenger of electrons photogenerated in TiO 2 particles which let down hole-electron recombination [26].
The doping with small amounts of metal impurities aims at facing one relevant drawback of TiO 2 , 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 TiO 2 , examined by the mineralization of oxalic acid under visible light irradiation, showed highest activity with 2% of Fe-doped TiO 2 [31].However, it was reported that the photocatalytic efficiency of metal doped TiO 2 strongly depends on the conditions of preparation, on the morphostructural 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 TiO 2 and irondoped TiO 2 with the sol-gel method and to study the efficiency of Fe-doped titania suspensions in the photocatalytic degradation of the gesaprim commercial herbicide under UV light, visible light and solar light.Gesaprim, herbicide used for the control of broadleaf weeds and some grassy weeds, contains atrazine as active compound and formulating agents as additional components.

Synthesis of Fe-Modified TiO 2 Photo Catalyst
The Fe-doped TiO 2 catalyst was prepared by the sol-gel method as below: A mixture of H 2 O (1.25 ml), HCl (0.

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 irradiation.Gesaprim degradation, using TiO 2 and Fe-doped TiO 2 suspension of the powders synthesized, was followed by measuring the chemical oxygen demand (COD) and Total organic carbon (TOC).The initial concentration of gesaprim which corresponds to its maxima solubility in water at pH 3 is 40 ppm.Such concentration has a COD and TOC content of 38 ppm and 20 ppm, respectively.The photocatalytic activity the photo catalyst Fe-doped TiO 2 was evaluated by measuring the decomposition rate of gesaprim in a circulation type reactor.
The reactor was similar to a laboratory glassware condenser (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 irradiation 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 polypropylene circulating pump (Cole Parmer).Stock solutions of gesaprim (0.106 g diluted in 2.5 L) were prepared with distilled water at pH 3 adjusted with H 2 SO 4 with gentle stirring during 2 d.The TiO 2 (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 intensity at the surface of the sample was 0.068 W/cm 2 ; whereas it varied inside of 0.059 -0.090 W/cm 2 range for the visible light.The gesaprim solution was in contact 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 concentration range of 0 -40 ppm.Samples were filtered as collected prior the analysis.

Characterization of Undoped TiO 2 and Iron Doped TiO 2 Powders
Figure 1 shows the XRD patterns of the TiO 2 and 0.5% Fe doped TiO 2 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 TiO 2 , the diffraction peaks for iron are completely missing from the XRD pattern for the doped iron TiO 2 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 TiO 2 [33], or by very fine dispersion of iron in the titania resulting in X-ray amorphous 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 TiO 2 powder [32].

Influence of Iron on the Photo Catalyst for the Degradation of Gesaprim under Visible Light
The photocatalytic activity of the Fe-doped TiO 2 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 TiO 2 photo catalyst (400 mg/L) under visible light irradiation (with 0.090 W/cm 2 using 20 W lamp).The highest initial degradation rate of gesaprim was found to be at 0.5% of iron content (by weight of TiO 2 ) achieving 95% of COD removal after 60 min of visible light irradiation (with an energy consumption 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 TiO 2 photo catalyst showed ~21% of COD removal at the same time under the same conditions.The degradation of gesaprim under solar irradiation (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 TiO 2 , 0.1% Fe-doped TiO 2 and 0.5% Fe-doped TiO 2 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 TiO 2 under solar light at 60 min of reaction.These results show that the increased activity of TiO 2 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- riers.Thus, the recombination rate of the electron-hole pairs is lowered and the photocatalytic activity is enhanced.It has been reported that the photocatalytic activity of iron doped TiO 2 strongly depends on the preparation 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 TiO 2 ).
It has also been reported that higher amount of Fe(III)doped TiO 2 than an optimal iron content is detrimental on the photocatalytic activity of the photo catalyst because the Fe(III) at high concentration in Fe-doped TiO 2 can act as a recombination centres [34].

Influence of Light Irradiation on the Photo Catalyst for the Degradation of Gesaprim
In order to study the effect of light source on the degradation of gesaprim by 0.5% Fe-doped TiO 2 catalyst, several experiments were carried out and recorded as a function of the energy consumption to compare the degradation 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 catalysis), 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   It is interesting to observe that the iron-doped TiO 2 enhanced the initial rate of COD removal under the solar irradiation with respect to that obtained with or without undoped TiO 2 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 catalysis 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 TiO 2 under visible irradiation (20 W lamp) after 60 minutes of reaction (with an energy consumption of 288 kJ/L).The high COD removals achieved is because the iron deposition on TiO 2 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 enhanced by the use of ultrasound in the presence of TiO 2 catalyst with COD removal of 84% at 150 minutes of UV light irradiation.The combination of these two processes (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 TiO 2 catalyst (under visible light) gives similar results at shorter time.
Figure 4(b) reports similar COD removal profiles under 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 TiO 2 powders to some extent because of an efficient charge separation of the UV light generated electron-hole pairs.

Conclusions
The gesaprim degradation rate depended on the iron content in the photo catalyst.The Fe-doped TiO 2 (0.5% by weight of TiO 2 ) showed higher TOC removal under visible light and was more active than the undoped TiO 2 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 degradation 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 TiO 2 particles which let down holeelectron recombination.
This advanced oxidation process offers an environmental alternative for the treatment of water polluted with this commercial herbicide.

Acknowledgements
This work was financially supported by the Programa de Mejoramiento del Profesorado (PROMEP) of the Secretaría 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.

Figure 2 .
Figure 2. COD profile for the degradation of gesaprim using TiO 2 and iron-doped TiO 2 (catalyst load: 400 ppm and pH 3) under (a) visible light (20 W) and (b) solar irradiation.

Figure 3 .
Figure 3. COD removal profile for the degradation of gesaprim using 400 ppm of 0.5% Fe-doped TiO 2 under different powers of the visible light.

Figure 4 .
Figure 4. Photocatalytic degradation of gesaprim using 400 ppm of 0.5% Fe-doped TiO 2 under different sources of light irradiation.oflight irradiation (Figure4) clearly show an improvment in the degradation of gesaprim by the presence of iron in the catalyst (0.5% Fe-doped TiO 2 ) under visible light from both the 20W Vis-lamp and the solar irradiation (Figure4(a)).It is interesting to observe that the iron-doped TiO 2 enhanced the initial rate of COD removal under the solar irradiation with respect to that obtained with or without undoped TiO 2 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 catalysis 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 TiO 2 under visible irradiation (20 W lamp) after 60 minutes of reaction (with an energy consumption of 288 kJ/L).The high COD removals achieved is because the iron deposition on TiO 2 surface improves the photocatalytic activity by suppressing the electron-hole recombination[36].