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Photodegradation of Triazine-Based Pesticides: Laser Flash Photolysis of s-Triazines

M. Canle L.a,b M.I. Fernández;a,b J.A. Santaballa;a S. Steenkenb

aDepartamento de Química Fundamental e Industrial. Universidade da Coruña. A Zapateira, s/n. E-15071 A Coruña.Galicia, SPAIN. E-mail: mcanle@udc.es bMax-Planck Institut für Strahlenchemie. Stiftstraße, 34-36. D-45470 Mülheim an der Ruhr, GERMANY.
Abstract.
As part of a project to clarify the mechanism of photodegradation of different pesticides under environmental-like conditions, we have studied the laser-flash photolysis of s-triazine itself and other four s-triazine derivatives commonly used as herbicides: Ametryn, Desmetryn, Prometryn and Terbutryn. All the studied compounds undergo monophotonic photoionization with very low quantum yields (fPI=0.07, 0.07, 0.13, 0.13 and 0.05, respectively) when the excitation is performed with 193 nm light. No reaction is observed with higher wavelength light. We conclude that, in the absence of photosensitizers, s-triazines are not photodegraded.
Introduction.
s-Triazines, six-membered aromatic rings with three carbons and three nitrogens alternating about the ring, are widely used in agriculture as herbicides.[1] In fact, in the last decade these compounds represented the second largely sold group of herbicides in the U.S.[2] Their herbicidal activity is due to their capability to inhibit certain reactions that are essential for photosynthesis.[3] The increasing usage of these compounds leads to a rise of their presence in natural waters.[4] Such problem is especially relevant in areas with permeable soils, due to the pollution of groundwater drinking resources.[5]
Despite these facts, little information is available on the photolysis of s-triazines under typical environmental conditions.[6] In contrast, there is growing public concern about the possible effects of such pollutants on human health and on the environment.[7] The fact that major monitoring studies were already performed long ago proofs such concern. Thus, Hörmann et al. monitored, already in 1976, the triazine herbicide residues in many different streams of Central Europe.[8] Methods of degradation like chlorination, ozonation, oxidation with H2O2 or ultrafiltration have been carried out for long.[9] Some studies on the products of photodegradation of s-triazines have also been performed.[10]ÑÑ
In the framework of a wider project aiming to clarify the mechanism of photodegradation of different pesticides in aqueous solution, we have studied the laser-flash photolysis of the triazine-based sulfur-containing herbicides Ametryn, Desmetryn, Prometryn, and Terbutryn. (1,3,5)-Triazine was also used as a model compound.
Experimental.
The s-triazines were of the highest purity commercially available (ca. 99 % ), and used without further purification. All other chemicals and gases were of the highest purity available. Water was obtained from a Millipore-Milli Q system.

The laser flash photolysis (LFP) experiments were carried out using: a) 193 nm (ArF*) excimer laser (Lambda Physik EMG150E), b) 248 nm (KrF*) excimer laser (Lambda Physik EMG103MSC), c) 266 nm Nd3+ YAG (JK Lasers System 2000) and ), d) 308 nm (XeCl* excimer laser (Lambda Physik EMG150E). Such devices provided unfocused 20 ns pulses (15 ns for the 266 nm Nd3+ YAG) with 10<E<80 mJ/pulse. Transmission filters were used to attenuate the intensity when necessary. The detection was optical, using a pulsed Xe lamp as analyzing light. The optical signals were digitized using Textronix transient recorders and interfaced to a computer that controlled the whole system. All solutions were deaerated and flown through Suprasil quartz cells. The LFP experiments were carried out at ca. 293±2 K. NaCl, [11]f was used for actinometry at 193 nm, the solutions being prepared with an absorbances at the excitation wavelength equal within 5% to that of the photolyzed substrate. Blank experiments of photolysis of Ar-saturated water showed no signal, confirming that the photoionization of water by 193 nm light is negligible. The extinction coefficient of the solvated electron, e(e-aq, 600 nm)=13300 M-1·cm-1 was used as a reference value to obtain the extinction coefficients of the organic radicals. [12]
Results and discussion.
LFP of the different triazines was carried out in aqueous solution of pH ca. 7 with 193, 248, 266 and 308 nm light (i.e.: photon energies of 6.40, 5.00, 4.67 and 4.03 eV). A photoionization process was observed when exciting with 193 nm light, and no reaction at all when the excitation was carried out with 248, 266 or 308 nm light. Figures 1 and 2 show, respectively, the time-resolved spectrum recorded after 193 nm LFP of an Ar-saturated and O2-saturated solutions of Ametryn.

Figure 1: time resolved spectrum following 193 nm LFP of Ar-satd. 50 µM Ametryn in 5% MeOH, pH ca. 7. Spectra recorded at the indicated times after the pulse.

Figure 2: time resolved spectrum following 193 nm LFP of O2-satd. 50 µM Ametryn in 5% MeOH, pH ca. 7. Spectra recorded at the indicated times after the pulse.

The intense band centered at 720 nm that appears in the Ar-saturated experiment (Figure 1) corresponds to e-aq.[13] In the presence of O2 (Figure 2), an excellent e-aq scavenger, such band disappears within 100 ns. The scavenging capacity of O2 is explained by the formation of O2*- through the process O2 + e-aq -> O2*-. The data available are not enough to identify the transient species absorbing at ca. 270 nm.
For all the studied compounds the 193 nm photoionization is a monophotonic process, as determined from [Delta]O.D. (e-aq) vs. (E / mJ) plots (shown in Figure 3). The photoionization quantum yields (fPI) were also obtained by comparing the yield of e-aq in the LFP of the different triazines with that of e-aq for NaCl actinometry solutions of the same optical density as the substrates at the excitation wavelength (fPI(Cl-)=0.41±0.08).? The obtained fPI values are compiled in Table 1. The very low values obtained suggest the existence of many effective decay pathways other than electron photoejection for the excited states generated upon photolysis to relax. It also appears as if the presence of bulky electron donating groups increases the ease of photoionization, which would be in agreement with a stabilization of the so-formed radical cation.

Figure 3: photonity of 193 nm LFP for the photoionization of Ar-saturated aqueous solutions of the different triazines studied.

Table 1: fPI observed upon 193 nm LFP or Ar-satd. solutions of the different used triazines.

Model

fPI

s-Triazine

0.005

Herbicides

fPI

Ametryn

0.071

Desmetryn

0.070

Prometryn

0.129

Terbutryn

0.129

In all cases the electron photoejection and electron hydration processes took place within the laser pulse, i.e.: in less than 20 ns. This is in agreement with previous observations of such processes taking place in less than 27 ps,[14] and would, in practice, mean that no ion-pair recombination takes place after the laser pulse.[15]
The results presented here allow us to predict that the photoionization threshold of triazine derivatives must be around 9.9 eV, of which 6.4 eV are provided by the exciting 193 nm photons and 3.5 eV come from the hydration of the photoionization products.[16]
On the basis of the experimental evidences available, the general mechanism proposed in Scheme 1 can be put forward for the far-UV induced photodegradation of triazines:

Scheme 1: general mechanism describing the far-UV photoionization of s-triazines.


From an environmental point of view, it is remarkable the fact that for all the studied triazines the only photo-initiated processes were observed when exciting with 193 nm light, which is not contained in the solar spectral irradiance.[17] This, in practice, means that s-triazine based herbicides do not undergo photodegradation upon exposition to sunlight.
Further research is in progress on photosensitization based degradation of s-triazines, as well as on the applications of radiation chemistry for the degradation of such compounds.
Conclusion.
Triazine-based herbicides are not photodegraded by sunlight. Instead, these compounds can be photodegraded (with low yields) by the action of far-UV radiation.
Acknowledgments.
M.I.F. acknowledges a Deutscher Akademischer Austauschdienst (DAAD, Germany) grant. MCL thanks the EU for a Training and Mobility of Researchers contract to work in the Max-Planck Institut für Strahlenchemie (Mülheim an der Ruhr, Germany), and for supporting a series of visits to the Paterson Institute for Cancer Research Free Radical Research Facility (Manchester, U.K.) within the Access to Large Scale Facilities activity. Thanks are also due to the Universidade da Coruña for different leaves of absence to MCL.
References.

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