**5. Photochemical degradation of triazine and phenylurea herbicides**

#### **5.1 Possible photoinitiated pathways for herbicide degradation**

An organic substrate may undergo the following photoinitiated reactions under natural sunlight or artificial source irradiation:


For a pollutant the processes given above are schematically visualized in Fig. 3.

#### **5.2 Direct sunlight photodegradation**

Direct sunlight photodegradation can proceed with substrates that are able to absorb the solar action spectrum. Solar radiation reaching the Earth´s surface has wavelengths ranging from about 300 nm upwards. Triazine and phenylurea compounds, which absorb at range well below 300 nm (absorption maxima at 220 – 235 nm) cannot therefore undergo direct sunlight photodegradation.

### **5.3 Homogeneous photocatalytic degradation in the presence of dissolved metal ions**

Homogeneous photocatalytic reactions of triazine herbicides in the presence of dissolved metal ions were studied for ferric, copper, and manganese ions (Klementova & Hamsova, 2000). Cupric and manganese (II) ions exhibited only small activities, and only in high concentrations. Table 2 shows the results for atrazine degradation in aqueous solutions under irradiation at a range of wavelengths from 300 to 350 nm. When no metal ions are added, no reaction occurs.

In the case of atrazine the addition of Cu (II) or Mn(II) ions results in conversion below 15 % or less. Ferric ions in comparable concentration cause the conversion of practically all the atrazine in 90 minutes of irradiation. The degradation of atrazine was shown to be strongly dependent on the ferric ion concentration (Fig. 4). Simazine and propazine did not show such a strong dependence on the added ferric ions.

A Critical View of the Photoinitiated Degradation of Herbicides 303

0 0 0 0 0 0 0 0 30 0 6 8 1 7 30 97 60 0 8 12 4 8 64 98 90 0 14 15 6 9 98 99 Table 2. Degradation of atrazine in photoinitiated reaction in air saturated aqueous solution in the presence of metal ions. Initial concentration of atrazine 5.0\*10-5 mol/l. Irradiation: Rayonet photochemical reactor RPR 100, lamps 3000Å, emission up to 290 nm filtered by

Cu(II) 1.0\*10-3 mol/l

0 20 40 60 80 100 **irradiation time (min)**

Fig. 4. Effect of ferric ions concentration on atrazine photochemical degradation (conditions of irradiation see Tab.2). Initial concentration of atrazine 5.0\*10-5 mol/l. (From Klementová

The photoreduction of ferric to ferrous ions occurs quickly under the irradiation of all three triazines, atrazine, propazine and simazine, though the reaction mixtures were saturated by the air. In the steady state, about 23% of added ferric ions are present in the reduced form in the reaction mixture of atrazine, about 70% in the reaction mixture of propazine, and nearly

time of irrad. (minutes)

0

& Hamsová, 2000).

90% in the reaction mixture of simazine.

20

40

60

**atrazine concentration (%)**

80

100

120

no added metal ions

Cu(II) 3.3\*10-4 mol/l

optical glass. (From Klementová & Hamsová, 2000.)

atrazine consumption (% of initial concentration)

> Mn(II) 1.6\*10-4 mol/l

Mn(II) 1.0\*10-3 mol/l

Fe(III) 1.0\*10-4 mol/l

Fe(III) 3.3\*10-4 mol/l

no Fe(III) added 1,5EXP-6 1,0EXP-5 6,6EXP-5 1,0EXP-4 1,6EXP-4 3,3EXP-4

Fig. 3. Scheme of possible degradation pathways of a pollutant non-absorbing solar radiation.

In order to prove the photocatalytic mechanism of the degradation in the triazine solutions, formation of Fe2+ ions was measured in the reaction system. The results are set out in Fig. 5.

Fig. 3. Scheme of possible degradation pathways of a pollutant non-absorbing solar

In order to prove the photocatalytic mechanism of the degradation in the triazine solutions, formation of Fe2+ ions was measured in the reaction system. The results are set out in Fig. 5.

radiation.


Table 2. Degradation of atrazine in photoinitiated reaction in air saturated aqueous solution in the presence of metal ions. Initial concentration of atrazine 5.0\*10-5 mol/l. Irradiation: Rayonet photochemical reactor RPR 100, lamps 3000Å, emission up to 290 nm filtered by optical glass. (From Klementová & Hamsová, 2000.)

Fig. 4. Effect of ferric ions concentration on atrazine photochemical degradation (conditions of irradiation see Tab.2). Initial concentration of atrazine 5.0\*10-5 mol/l. (From Klementová & Hamsová, 2000).

The photoreduction of ferric to ferrous ions occurs quickly under the irradiation of all three triazines, atrazine, propazine and simazine, though the reaction mixtures were saturated by the air. In the steady state, about 23% of added ferric ions are present in the reduced form in the reaction mixture of atrazine, about 70% in the reaction mixture of propazine, and nearly 90% in the reaction mixture of simazine.

A Critical View of the Photoinitiated Degradation of Herbicides 305

The photocatalytic reaction occurs in the stage where the reactants are absorbed on the catalyst surface, the activation of the reaction being photonic activation. The semiconductor is activated by irradiation from a light source of appropriate wavelength depending on the band gap energy of the semiconductor. The activation generates a pair of charge carriers, a hole, *h+*, and an electron, *e-*; the charge carriers generated photochemically can react with

Fig. 6. Scheme of oxidative species production on semiconductors under irradiation.

self-cleaning properties (Devilliers, 2006).

Various metal oxides, e.g. TiO2 (Hashimoto et al, 2005; Héquet et al., 2001; Konstantinou et al., 2001a; Linsebigler et al., 1995; Pelizzetti et al, 1990; Penuela& Barceló, 2000) ZnO (Byrappa et al., 2006), CeO2 (Yongging Zha et al., 2007), ZrO2 (Bota et al., 1999), WO3 (Guo et al., 2007) and many other composites of semiconductors or doped semiconductors have been used as catalysts in semiconductor photocatalytic reactions (e.g. Dunliang et al. , 2009). TiO2 – the most widely used semiconductor in contaminant photocatalysis – occurs in three distinct polymorphs: anatase, rutile and brookite. Of these three forms only anatase is functional as a photocatalyst. Anatase is a typical n-type semiconductor with a band gap of about 3.2 eV. Photons with a wavelength shorter then 385 nm have enough energy to excite electrons from the valence band to the conduction band of this material. Since the 1970s, anatase has been a popular choice as semiconductor photocatalyst in research efforts because it is non-toxic and mechanically stable, has high photo-activity and low cost, and exhibits a reasonable overlap with the ultra-violet portion of the solar spectrum which makes it attractive for solar applications. Up to now a multitude of compounds have been investigated as target pollutants in photocatalytic oxidation studies on TiO2. The studies have been performed at bench scale using small reactors operating as batch or flow reactor systems. Besides pollutant degradation successful tests for the treatment of bacteria, viruses, fungi, and tumor cells have been reported. Construction materials coated with TiO2 exhibit

Triazine herbicides photocatalytic degradation on TiO2 has been studied by several authors, e.g. Héquet et al., 2001; Konstantinou et al., 2001;Pelizzetti et al., 1990; Penueala & Barceló,

molecules on the surface of the semiconductor (Eqs. 6 – 11 and Fig. 6).

Fig. 5. Photochemical reduction of Fe(III) in the reaction systems with atrazine, propazine and simazine, resp., in the air saturated reaction mixtures. Concentration of substrates 5.0\*10-5 mol/l, concentration of initial Fe3+ ions 1.0\*10-4 mol/l. Conditions of irradiation – see Table 2. (From Klementová & Hamsová, 2000).

Homogeneous photocatalytic reactions in the presence of ferric ions may provide a possible pathway for the photochemical degradation of atrazine in water bodies; the problem being that the iron content in natural surface waters is about 1\*10-5 mol/l, a relatively ineffective concentration for atrazine degradation. Other triazine derivatives, propazine and simazine, seem not to be affected by homogeneous photocatalytic degradation in the presence of the ions that are most abundant in natural waters (iron and manganese).

#### **5.4 Heterogeneous photocatalytic degradation**

There are no data on the heterogeneous photocatalytic degradation of herbicides with particulate matter in natural waters. Ample studies deal on the other hand with heterogeneous photochemical degradation in relation to semiconductors especially in the context of decontamination option for drinking water and in waste-water treatment.

Semiconductor photocatalysis uses solid catalytic systems where five discrete stages associated with conventional heterogeneous catalysis can be distinguished:


atrazine propazine simazine

0 20 40 60 80 100 **irradiation time (min)**

Fig. 5. Photochemical reduction of Fe(III) in the reaction systems with atrazine, propazine and simazine, resp., in the air saturated reaction mixtures. Concentration of substrates 5.0\*10-5 mol/l, concentration of initial Fe3+ ions 1.0\*10-4 mol/l. Conditions of irradiation – see

Homogeneous photocatalytic reactions in the presence of ferric ions may provide a possible pathway for the photochemical degradation of atrazine in water bodies; the problem being that the iron content in natural surface waters is about 1\*10-5 mol/l, a relatively ineffective concentration for atrazine degradation. Other triazine derivatives, propazine and simazine, seem not to be affected by homogeneous photocatalytic degradation in the presence of the

There are no data on the heterogeneous photocatalytic degradation of herbicides with particulate matter in natural waters. Ample studies deal on the other hand with heterogeneous photochemical degradation in relation to semiconductors especially in the

Semiconductor photocatalysis uses solid catalytic systems where five discrete stages

a. transfer of liquid or gaseous phase reactant to the catalytic surface by the diffusion;

context of decontamination option for drinking water and in waste-water treatment.

associated with conventional heterogeneous catalysis can be distinguished:

e. removal of products from the interface region by the diffusion.

Table 2. (From Klementová & Hamsová, 2000).

**5.4 Heterogeneous photocatalytic degradation** 

b. adsorption of the reactant on the catalyst surface;

c. reaction of the adsorbed molecules;

d. desorption of products;

ions that are most abundant in natural waters (iron and manganese).

**reduced Fe (%)**

The photocatalytic reaction occurs in the stage where the reactants are absorbed on the catalyst surface, the activation of the reaction being photonic activation. The semiconductor is activated by irradiation from a light source of appropriate wavelength depending on the band gap energy of the semiconductor. The activation generates a pair of charge carriers, a hole, *h+*, and an electron, *e-*; the charge carriers generated photochemically can react with molecules on the surface of the semiconductor (Eqs. 6 – 11 and Fig. 6).

Fig. 6. Scheme of oxidative species production on semiconductors under irradiation.

Various metal oxides, e.g. TiO2 (Hashimoto et al, 2005; Héquet et al., 2001; Konstantinou et al., 2001a; Linsebigler et al., 1995; Pelizzetti et al, 1990; Penuela& Barceló, 2000) ZnO (Byrappa et al., 2006), CeO2 (Yongging Zha et al., 2007), ZrO2 (Bota et al., 1999), WO3 (Guo et al., 2007) and many other composites of semiconductors or doped semiconductors have been used as catalysts in semiconductor photocatalytic reactions (e.g. Dunliang et al. , 2009).

TiO2 – the most widely used semiconductor in contaminant photocatalysis – occurs in three distinct polymorphs: anatase, rutile and brookite. Of these three forms only anatase is functional as a photocatalyst. Anatase is a typical n-type semiconductor with a band gap of about 3.2 eV. Photons with a wavelength shorter then 385 nm have enough energy to excite electrons from the valence band to the conduction band of this material. Since the 1970s, anatase has been a popular choice as semiconductor photocatalyst in research efforts because it is non-toxic and mechanically stable, has high photo-activity and low cost, and exhibits a reasonable overlap with the ultra-violet portion of the solar spectrum which makes it attractive for solar applications. Up to now a multitude of compounds have been investigated as target pollutants in photocatalytic oxidation studies on TiO2. The studies have been performed at bench scale using small reactors operating as batch or flow reactor systems. Besides pollutant degradation successful tests for the treatment of bacteria, viruses, fungi, and tumor cells have been reported. Construction materials coated with TiO2 exhibit self-cleaning properties (Devilliers, 2006).

Triazine herbicides photocatalytic degradation on TiO2 has been studied by several authors, e.g. Héquet et al., 2001; Konstantinou et al., 2001;Pelizzetti et al., 1990; Penueala & Barceló,

A Critical View of the Photoinitiated Degradation of Herbicides 307

The photosensitized degradation of triazine and phenylurea herbicide in the presence of humic substances has been studied by several authors, e.g. Amine-Khodja et al. (2006); Comber (1999); Gerecke et al. (2001); Klementova & Piskova (2005); Konstantinou et al. (2001b), Minero et al. (1992) and Schmitt et al. (1995). The results suggest that there is no unambiguous answer about the influence of humic substances. Some authors report better degradation of the substrates, other report decrease in reaction rates in the presence of humic substances. The explanation probably lies in the combination of absorption characteristics of humic samples, their concentrations and the light sources used in the studies. In concentrated humic waters, inner filtration (i. e. the absorption of a significant part of the radiation energy by the photosensitively inactive parts of humic molecules) may play an important role and cause a decrease in the reaction rate of degradation. The heterogeneous chemical character of humic fractions may also be responsible for the

Two groups of artificial sensitizers which provide defined oxidative species were studied in our group for triazine and triazine metabolite degradation: phthalocyanines, i.e. photosensitizers providing singlet oxygen, and anthraquinonesulfonate causing formation of superoxide anions (Klementová & Hamsová, 2000). To our surprise phthalocyanines (aluminium-chloro-phthalocynanine-disulfonate and zinc-phthalocyanine-trisulfonate) showed no observable effect. Anthraquinonesulfonate presence in the aqueous solutions of triazine herbicides (atrazine, propazine, simazine) and the two of atrazine metabolites (desethylatrazine and desisopropylatrazine) resulted in a relatively swift degradation (Fig. 8). Anthraquinonesulfonate was repeatedly added to the reaction mixtures since its molecules are degraded by UV light. This result suggests that triazine herbicides are readily degradable by superoxide species. Nevertheless, the aromatic ring is not broken down so

the decomposition is incomplete as it is in other sensitized and catalyzed reactions.

Fig. 7. Structure of fulvic acids. (From Dojlido & Best, 1993).

variable photosensitizing activities of individual humic samples.

2000), in some cases with the addition of oxidative species such as hydrogen peroxide or photo-Fenton system, H2O2/Fe(III), providing hydroxyl radicals. Atrazine was found to be degraded to desethylatrazine and desisopropylatrazine, i. e. the same compounds that are metabolites of biodegradation. These metabolites are not easily further degraded in the photocatalytic process on TiO2.

In our group (Klementová, 2011), we compared the degradation of atrazine in the homogeneous photocatalytic reaction in the presence of Fe (III) and the photocatalytic degradation on TiO2 (batch experiment, glass coated with TiO2, irradiation by Philips TLD 15 W 08 lamps). The reaction constant of the heterogeneous photocatalytic reaction (0.018 min-1) was comparable with the reaction constant in reaction mixtures with higher concentrations of ferric ions (0.021 min-1 for Fe(III) concentration 1.4\*10-4 mol/l).

The degradation of phenylurea herbicides on TiO2 has been studied e.g. by Amorisco et al.(2006), Haque et al. (2006) and Lhomme et al. (2005). The results of such studies show the importance of operational conditions (adsorption capacity, initial concentrations chlorotoluron, TiO2 forms – coated or in suspension (Lhomme et al., 2005). The pathway of chlorotoluron degradation contained a substitution of chloride ion by the hydroxyl group on the aromatic ring, the demethylation of N group on the side chain, and in some cases a breaking down of the aromatic ring was observed.

Heterogeneous photocatalysis may represent a feasible pathway for the degradation of herbicides in waste-water treatment or even drinking water treatment, especially under conditions where the aromatic ring structure is broken down.

#### **5.5 Photosensitized reactions**

Photosensitized reactions may proceed in natural waters in the presence of natural sensitizers such as humic substances. Humic substances originate from the decay of plant and animal biomass and humification reactions in the decaying material. The molecules of humic substances are of variable structure and size (molecular weight ranging from several hundreds to several hundreds of thousands). Humic substances are classified into three operational classes:


Humic acids and fulvic acids have an acidic character due to their substential content of carboxylic and phenolic functional groups (Schnitzer & Khan, 1972); Dojlido & Best, 1993). The basic structural features of humic and fulvic acids are shown in Fig. 7.

Humic and fulvic acids have featureless absorption spectra with increasing absorption from the short-wavelengths of visible light through the ultraviolet radiation range.

Photosensitizing properties resulting in the production of singlet oxygen molecules (1O2), superoxide anions (O2 -), hydroxyl radicals (HO•), peroxyradicals (ROO•), and hydrated electrons (eaq -) have been well established (Cooper et al., 1989; Hoigné et al., 1989; Mill T., 1989; Simmons & Zepp, 1986).

2000), in some cases with the addition of oxidative species such as hydrogen peroxide or photo-Fenton system, H2O2/Fe(III), providing hydroxyl radicals. Atrazine was found to be degraded to desethylatrazine and desisopropylatrazine, i. e. the same compounds that are metabolites of biodegradation. These metabolites are not easily further degraded in the

In our group (Klementová, 2011), we compared the degradation of atrazine in the homogeneous photocatalytic reaction in the presence of Fe (III) and the photocatalytic degradation on TiO2 (batch experiment, glass coated with TiO2, irradiation by Philips TLD 15 W 08 lamps). The reaction constant of the heterogeneous photocatalytic reaction (0.018 min-1) was comparable with the reaction constant in reaction mixtures with higher

The degradation of phenylurea herbicides on TiO2 has been studied e.g. by Amorisco et al.(2006), Haque et al. (2006) and Lhomme et al. (2005). The results of such studies show the importance of operational conditions (adsorption capacity, initial concentrations chlorotoluron, TiO2 forms – coated or in suspension (Lhomme et al., 2005). The pathway of chlorotoluron degradation contained a substitution of chloride ion by the hydroxyl group on the aromatic ring, the demethylation of N group on the side chain, and in some cases a

Heterogeneous photocatalysis may represent a feasible pathway for the degradation of herbicides in waste-water treatment or even drinking water treatment, especially under

Photosensitized reactions may proceed in natural waters in the presence of natural sensitizers such as humic substances. Humic substances originate from the decay of plant and animal biomass and humification reactions in the decaying material. The molecules of humic substances are of variable structure and size (molecular weight ranging from several hundreds to several hundreds of thousands). Humic substances are classified into three

Humic acids and fulvic acids have an acidic character due to their substential content of carboxylic and phenolic functional groups (Schnitzer & Khan, 1972); Dojlido & Best, 1993).

Humic and fulvic acids have featureless absorption spectra with increasing absorption from

Photosensitizing properties resulting in the production of singlet oxygen molecules (1O2), superoxide anions (O2 -), hydroxyl radicals (HO•), peroxyradicals (ROO•), and hydrated electrons (eaq -) have been well established (Cooper et al., 1989; Hoigné et al., 1989; Mill T.,

concentrations of ferric ions (0.021 min-1 for Fe(III) concentration 1.4\*10-4 mol/l).

photocatalytic process on TiO2.

**5.5 Photosensitized reactions** 

1989; Simmons & Zepp, 1986).

operational classes:

breaking down of the aromatic ring was observed.

conditions where the aromatic ring structure is broken down.


The basic structural features of humic and fulvic acids are shown in Fig. 7.

the short-wavelengths of visible light through the ultraviolet radiation range.


Fig. 7. Structure of fulvic acids. (From Dojlido & Best, 1993).

The photosensitized degradation of triazine and phenylurea herbicide in the presence of humic substances has been studied by several authors, e.g. Amine-Khodja et al. (2006); Comber (1999); Gerecke et al. (2001); Klementova & Piskova (2005); Konstantinou et al. (2001b), Minero et al. (1992) and Schmitt et al. (1995). The results suggest that there is no unambiguous answer about the influence of humic substances. Some authors report better degradation of the substrates, other report decrease in reaction rates in the presence of humic substances. The explanation probably lies in the combination of absorption characteristics of humic samples, their concentrations and the light sources used in the studies. In concentrated humic waters, inner filtration (i. e. the absorption of a significant part of the radiation energy by the photosensitively inactive parts of humic molecules) may play an important role and cause a decrease in the reaction rate of degradation. The heterogeneous chemical character of humic fractions may also be responsible for the variable photosensitizing activities of individual humic samples.

Two groups of artificial sensitizers which provide defined oxidative species were studied in our group for triazine and triazine metabolite degradation: phthalocyanines, i.e. photosensitizers providing singlet oxygen, and anthraquinonesulfonate causing formation of superoxide anions (Klementová & Hamsová, 2000). To our surprise phthalocyanines (aluminium-chloro-phthalocynanine-disulfonate and zinc-phthalocyanine-trisulfonate) showed no observable effect. Anthraquinonesulfonate presence in the aqueous solutions of triazine herbicides (atrazine, propazine, simazine) and the two of atrazine metabolites (desethylatrazine and desisopropylatrazine) resulted in a relatively swift degradation (Fig. 8). Anthraquinonesulfonate was repeatedly added to the reaction mixtures since its molecules are degraded by UV light. This result suggests that triazine herbicides are readily degradable by superoxide species. Nevertheless, the aromatic ring is not broken down so the decomposition is incomplete as it is in other sensitized and catalyzed reactions.

A Critical View of the Photoinitiated Degradation of Herbicides 309

Photolytic degradation of triazine and phenylurea herbicides has been studied by several authors. Frimmel & Hessler (1994) irradiated atrazine, desethyatrazine and simazine by low pressure mercury lamp. The rate constants of individual reaction were identical (1.9\*10-4 s-1). Palm & Zetzsch (1996) carried out kinetic experiments with atrazine, propazine and simazine irradiated by xenon lamp in quartz vessels. Their kinetic evalutation gave the rate constants similar to those calculated by Frimmel & Hessler (1994); slightly higher rate constants and differing for the individual substrates studied were gained by Klementová & Píšková (2005) who irradiated atrazine, simazine, propazine, desethylatrazine and desisopropylatrezine by RPR 3000Å lamps (wavelength range 250 – 350 nm) – see Table 3.

**triazine** atrazine propazine simazine DEA DIPA

Table 3. First-order kinetics rate constant for photolytic UV degradation (lamps RPR 3000Å) of triazine and triazine derivatives. DEA – desethylatrazine; DIPA – desisopropylatrazine. Phenylurea herbicides UV photolysis has been studied e.g. by Benitez et al. (2006) for chlorotoluron, diuron, isoproturon, and by Klementová & Zemanová (2008) for chlorotoluron. Benitez et al. (2006) reported a dependence of the reaction rate on the pH value of the solution; the results published by Klementova & Zemanová (2008) did not support the reported pH dependence, the degradation was pH independent in the range of

Measuring the content of dissolved organic carbon (DOC) by DOC analyzer revealed that photolysis in solutions saturated with air results in the partial mineralization of organic substrates, i.e. decomposition of the organic carbon into CO2. About 20 % of organic carbon

Photolytic degradation by short-wavelength radiation therefore apparently represents a powerful tool for herbicides degradation in waste-water and drinking water treatment, since

In all cases where photochemical degradation was observed in our experiments, the initial step of the degradation of the triazine and phenylurea herbicides and triazine herbicide metabolites was dechlorination and hydroxyderivative formation. Chlorine was found in the solution as chloride ions, Cl-, that were detected in the reaction mixtures by ion chromatography. Hydroxyderivatives were detected by high performance liquid chromatography with a mass spectrometer as an analyzer. Fig. 9 shows one example of herbicide (chlorotolurone) degradation, and chloride ions and hydroxyderivative formation. In this case, as well as in the case of other triazine substrates, the plots of the substrate decomposition and the chloride formation are perfectly symmetrical. Hydroxyderivatives are intermediates that decompose further with a reaction rate constant nearly equal to that

**5.7 Photochemical degradation of triazine and phenylurea herbicides – common** 

4.64\*10-4 4.35\*10-4 5.45\*10-4 5.86\*10-4 6.33\*10-4

**rate constant (s-1)** 

pH values from 2 to 11.

**features** 

was mineralized in 90 minutes of irradiation.

it leads to total decomposition of organic matter.

of the original substrate decomposition.

#### **5.6 Photolytic degradation by short-wavelength radiation**

Direct photolytic degradation is a decomposition that follows the absorption of a photon (and therefore a rearrangement in the electron density distribution of the molecule in the excited state). The reaction includes only one reactant, i.e. the molecule that undergoes photolysis. The products of a photolytic splitting may undergo another photolytic decompositon if the radiation is of a suitable wavelength. The reaction follows the first order kinetics scheme (Eq. 12).

Fig. 8. Photosensitized degradation of triazine herbicides atrazine, simazine and propazine, and atrazine metabolites desethylatrazine (DEA) and desisopropylatrazine (DIPA) with anthraquinone sulfonate as the sensitizer. Initial concentration of individual substrates: 5.0\*10-5 mol/l. Concentration of anthraquinonesulphonate after addition: 1\*10-4 mol/l, addiotions each 30 minutes. Irradiation: Rayonet photochemical reactor RPR 100, lamps 3000Å, emission up to 290 nm filtered by optical glass. (From Klementová & Hamsová, 2000).

$$\mathbf{A} \to \mathbf{B} \tag{12}$$

To achieve a photolytic decomposition highly energetic radiation is necessary. Usually a low pressure mercury lamp (emitting most radiation energy at the 254 nm wavelength) is used in these experiments. It is therefore obvious that such processes cannot contribute to herbicide degradation on the Earth´s surface, but have their potential in waste-water and drinking water treatment.

Direct photolytic degradation is a decomposition that follows the absorption of a photon (and therefore a rearrangement in the electron density distribution of the molecule in the excited state). The reaction includes only one reactant, i.e. the molecule that undergoes photolysis. The products of a photolytic splitting may undergo another photolytic decompositon if the radiation is of a suitable wavelength. The reaction follows the first

> atrazine simazine propazine DIPA DEA

0 50 100

Fig. 8. Photosensitized degradation of triazine herbicides atrazine, simazine and propazine, and atrazine metabolites desethylatrazine (DEA) and desisopropylatrazine (DIPA) with anthraquinone sulfonate as the sensitizer. Initial concentration of individual substrates: 5.0\*10-5 mol/l. Concentration of anthraquinonesulphonate after addition: 1\*10-4 mol/l, addiotions each 30 minutes. Irradiation: Rayonet photochemical reactor RPR 100, lamps 3000Å, emission up to 290 nm filtered by optical glass. (From Klementová & Hamsová,

 A → B (12) To achieve a photolytic decomposition highly energetic radiation is necessary. Usually a low pressure mercury lamp (emitting most radiation energy at the 254 nm wavelength) is used in these experiments. It is therefore obvious that such processes cannot contribute to herbicide degradation on the Earth´s surface, but have their potential in waste-water and

**irradiation (time)**

**5.6 Photolytic degradation by short-wavelength radiation** 

order kinetics scheme (Eq. 12).

0

20

40

**c o n c e n t r a t i o n ( % )**

2000).

drinking water treatment.

60

80

100

120

Photolytic degradation of triazine and phenylurea herbicides has been studied by several authors. Frimmel & Hessler (1994) irradiated atrazine, desethyatrazine and simazine by low pressure mercury lamp. The rate constants of individual reaction were identical (1.9\*10-4 s-1). Palm & Zetzsch (1996) carried out kinetic experiments with atrazine, propazine and simazine irradiated by xenon lamp in quartz vessels. Their kinetic evalutation gave the rate constants similar to those calculated by Frimmel & Hessler (1994); slightly higher rate constants and differing for the individual substrates studied were gained by Klementová & Píšková (2005) who irradiated atrazine, simazine, propazine, desethylatrazine and desisopropylatrezine by RPR 3000Å lamps (wavelength range 250 – 350 nm) – see Table 3.


Table 3. First-order kinetics rate constant for photolytic UV degradation (lamps RPR 3000Å) of triazine and triazine derivatives. DEA – desethylatrazine; DIPA – desisopropylatrazine.

Phenylurea herbicides UV photolysis has been studied e.g. by Benitez et al. (2006) for chlorotoluron, diuron, isoproturon, and by Klementová & Zemanová (2008) for chlorotoluron. Benitez et al. (2006) reported a dependence of the reaction rate on the pH value of the solution; the results published by Klementova & Zemanová (2008) did not support the reported pH dependence, the degradation was pH independent in the range of pH values from 2 to 11.

Measuring the content of dissolved organic carbon (DOC) by DOC analyzer revealed that photolysis in solutions saturated with air results in the partial mineralization of organic substrates, i.e. decomposition of the organic carbon into CO2. About 20 % of organic carbon was mineralized in 90 minutes of irradiation.

Photolytic degradation by short-wavelength radiation therefore apparently represents a powerful tool for herbicides degradation in waste-water and drinking water treatment, since it leads to total decomposition of organic matter.

#### **5.7 Photochemical degradation of triazine and phenylurea herbicides – common features**

In all cases where photochemical degradation was observed in our experiments, the initial step of the degradation of the triazine and phenylurea herbicides and triazine herbicide metabolites was dechlorination and hydroxyderivative formation. Chlorine was found in the solution as chloride ions, Cl-, that were detected in the reaction mixtures by ion chromatography. Hydroxyderivatives were detected by high performance liquid chromatography with a mass spectrometer as an analyzer. Fig. 9 shows one example of herbicide (chlorotolurone) degradation, and chloride ions and hydroxyderivative formation. In this case, as well as in the case of other triazine substrates, the plots of the substrate decomposition and the chloride formation are perfectly symmetrical. Hydroxyderivatives are intermediates that decompose further with a reaction rate constant nearly equal to that of the original substrate decomposition.

A Critical View of the Photoinitiated Degradation of Herbicides 311


Nevertheless, environmental pollution including water and soil pollution with herbicides is an increasingly grave problem, and with herbicides resistent to biodegradation and persisting for a long time in the environment the possibilities of photochemical degradation will not cease to attract attention. The possibilities for further development are open especially in the area of heterogeneous photocatalysis. An important key to success will be the utilisation of nano-sized photocatalyst powders dispersed on substrates with extremely large surface areas. Another approach is the modification of TiO2 to make it sensitive to visible light. So far the researchers investigating in this field are struggling with the issue of low reproducibility and chemical stability, nonetheless heterogeneous photocatalysis

I would like to thank my son David Klement for his help with formulas and schemes

Amine-Khodja A., Trubetskaya O. Trubetskoj O., Cavani L., Ciavatta C., Guyot G. & Richard

Amorisco A., Losito I., Carbonara T., Palmisano F. & Zamboni P.G. (2006). Photocatalytic

Badawi N., Rønhede S., Olsson S., Kragelund B. B., Johnsen A.H., Jacobsen O.S. & Aamand

Behki R.M. & Khan S.U. (1986). Degradation of Atrazine by Pseudomonas: N-dealkylation

Behki R.M. & Khan S.U. (1994). Degradation of atrazine, propazine and simazine by Rhodococcus Strain B-30. *J. Agric. Food. Chem.*, Vol. 42, pp. 1237 – 1241. Botta S.G., Navío J.A., Hidalgo M.C., Restrepo G.M & Litter M.J. (1999). Photocatalytic

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photocatalyst surface.

**7. Acknowledgement** 

– 1027.

Vol. 20, pp. 1569 – 1576.

10, pp. 2806 – 2812.

pp. 746 – 749.

drawing.

**8. References** 

be taken into consideration.

represents a promising prospect for 21 century.

of light energy to enable purification of higher columns of solutions. With heterogenous photocatalysis the three-dimensionality has one more aspect: the photocatalytic reaction on a semiconductor is a surface process, thus the reactant must be captured by the

Fig. 9. Chloride ion release and hydroxyderivative formation in chlorotoluron photodecomposition.
