**2. Photoinitiated reactions**

Each reaction started by an absorption of radiation may be classified as a photochemical or photoinitiated reaction. According to the mechanism of the photoinitiated reaction, photolytic, photosensitized and photocatalytic reactions can be distinguished.

A photolytic reaction is usually understood as a reaction in which the quantum of radiation absorbed has enough energy to cause the breaking of a covalent bond in the substrate compound. Usually highly energetic UV radiation (less than 250 nm) is necessary for this purpose. These reactions cannot proceed on the Earth´s surface since solar radiation reaching the Earth´s surface contains wavelengths greater than 290 nm.

A photosensitized reaction needs a sensitizer molecule. This is a molecule that is able to absorb radiation and to transfer the absorbed excitation energy onto another molecule. The

A Critical View of the Photoinitiated Degradation of Herbicides 299

S-triazine herbicides contain an aromatic ring with three N heteroatoms. The formula of a triazine herbicide, atrazine, is shown in Fig. 1., the formula of a phenylurea herbicide,

The triazine herbicides were introduced in the 1950s (Gysin & Knüsli, 1957, Gast et al., 1956, both in Tomlin, 2003), phenylurea pesticides a decade later (L´Hermite et al., 1969, in

The solubilities of these herbicides in water are in milligrams or at most tens of milligrams per liter as shown for three trazine and one phenylurea herbicide in Table 1. Table 1 also summarizes the DT50 values for the selected herbicides. DT50 signifies 50% dissipation time, i.e. the amount of time required for 50% of the initial pesticide concentration to dissipate.

> field: 16 – 77, median 41 natural waters: 10 –105 groundwaters: 105 - >200

water: >200

**3. Characterisation of s-triazine and phenylurea herbicides** 

Fig. 1. The structural formula of a triazine herbicide, atrazine.

Fig. 2. The structural formula of a phenylurea herbicide, chlorotoluron .

Unlike half-life dissipation time does not assume a specific degradation model.

Herbicide solubility (mg/l) DT50 (days)

Propazine 5.0 (20°C soil: 80 - 100 Simazine 6.2 (20°C) soil: 27 - 102 chlorotoluron 74 (25°C) soil: 30- 40

Table 1. Solubilities and DT50 values of selected triazine and phenylurea herbicides as given

All these herbicides are photosynthetic electron transport inhibitors at the photosystem II receptor site. They are all also systemic herbicides. Systemic herbicides (in comparison with contact herbicides) are translocated through the plant, either from foliar application down to the roots or from soil application up to the leaves. They are capable of controlling perennial plants and may be slower in action but ultimitaly more effective than contact

Atrazine 33 (22°C)

chlorotoluron, in Fig. 2.

Tomlin 2003).

in Tomlin (2003).

herbicides.

energy can be transferred either onto an organic molecule, substrate (e.g. herbicide molecule), or onto an oxygen molecule as shown in Eqs. 1 - 5.

$$\text{!Sens} + \text{hv} \to \text{!Sens}^\* \tag{1}$$

1Sens\* + 1Substrate → 1Substrate\* + 1Sens → Product + 1Sens (2)

1Sens\* → *through ISC* → 3Sens\* (3)

$$\text{\textquotedblleft Serss}^\* + \text{\textquotedblright} \text{\textquotedblright} \text{\textquotedblleft O}\_2 \to \text{\textquotedblleft O}\_2 \tag{4}$$

$$\text{'O}\_2 + \text{'Ssubstrate} \rightarrow \text{Oxidized product} \tag{5}$$

Eq.1 represents excitation of the sensitizer from the ground state (which is always a singlet state, i.e. all electrons in the molecule are paired) to the first excited singlet state. Eq. 2 represents energy transfer onto the substrate and its subsequent reaction into a product. Eq. 3 represents the conversion of the sensitizer from the first excited singlet state (all electrons are paired in the molecule in a singlet state) into the first triplet state (where two electrons are unpaired) through so called intersystem crossing (ISC). The sensitizer in the triplet state is able to react with molecular oxygen dissolved in the reaction mixture (Eq.4) because the ground state of molecular oxygen with its two unpaired electrons is a triplet state. If this ISC process did not occur, the reaction would not proceed since a reaction between a singlet and a triplet state molecule is spin-forbidden. The reaction results in the formation of a powerful oxidative species, singlet oxygen that oxidizes organic substrate molecules (Eq. 5).

Photocatalysis may occur as a homogeneous process or as a heterogeneous process. In homogeneous photocatalytic reactions light produces a catalytically active form of a catalyst. E. g. ferric ions may be reduced photochemically in the presence of an electron donor to ferrous ions that exhibit much higher catalytic activity. The subsequent catalytic reaction of a substrate is a ´dark´ reaction, i.e. not photochemical, since the reaction does not need light. Heterogeneous photocatalysis includes photochemical reactions on semiconductors. It proceeds via the formation of an electron-hole pairs under irradiation. These holes and electrons react with the solvent (water) and dissolved oxygen to produce an oxidative species, mainly OH radicals (Eqs. 6 – 11).

$$\rm H^{+} + H\_{2}O \rightarrow HO^{+} + H^{+} \tag{6}$$

$$\text{h}^\* + \text{OH} \cdot \rightarrow \text{HO}^\* \tag{7}$$

$$\rm O\_2 + e^- \rightarrow \rm O\_2\*\tag{8}$$

$$\rm O\_2^{\ast \cdot} + H^+ \to \rm HO\_2^{\ast} \tag{9}$$

$$\rm{2HO\_2^\*} \rightarrow \rm{H\_2O\_2} + \rm{O\_2} \tag{10}$$

 H2O2 + O2▪- → HO▪ + O2 + OH- (11)

energy can be transferred either onto an organic molecule, substrate (e.g. herbicide

1Sens + h → 1Sens\* (1)

1Sens\* + 1Substrate → 1Substrate\* + 1Sens → Product + 1Sens (2)

1Sens\* → *through ISC* → 3Sens\* (3)

3Sens\* + 3O2 → 1O2 (4)

 1O2 + 1Substrate → Oxidized product (5) Eq.1 represents excitation of the sensitizer from the ground state (which is always a singlet state, i.e. all electrons in the molecule are paired) to the first excited singlet state. Eq. 2 represents energy transfer onto the substrate and its subsequent reaction into a product. Eq. 3 represents the conversion of the sensitizer from the first excited singlet state (all electrons are paired in the molecule in a singlet state) into the first triplet state (where two electrons are unpaired) through so called intersystem crossing (ISC). The sensitizer in the triplet state is able to react with molecular oxygen dissolved in the reaction mixture (Eq.4) because the ground state of molecular oxygen with its two unpaired electrons is a triplet state. If this ISC process did not occur, the reaction would not proceed since a reaction between a singlet and a triplet state molecule is spin-forbidden. The reaction results in the formation of a powerful oxidative species, singlet oxygen that oxidizes organic substrate

Photocatalysis may occur as a homogeneous process or as a heterogeneous process. In homogeneous photocatalytic reactions light produces a catalytically active form of a catalyst. E. g. ferric ions may be reduced photochemically in the presence of an electron donor to ferrous ions that exhibit much higher catalytic activity. The subsequent catalytic reaction of a substrate is a ´dark´ reaction, i.e. not photochemical, since the reaction does not need light. Heterogeneous photocatalysis includes photochemical reactions on semiconductors. It proceeds via the formation of an electron-hole pairs under irradiation. These holes and electrons react with the solvent (water) and dissolved oxygen to produce an

h+ + H2O → HO▪ + H+ (6)

h+ + OH- → HO▪ (7)

O2▪- + H+ → HO2▪ (9)

H2O2 + O2▪- → HO▪ + O2 + OH- (11)

→ O2▪- (8)

▪ → H2O2 + O2 (10)

molecule), or onto an oxygen molecule as shown in Eqs. 1 - 5.

molecules (Eq. 5).

oxidative species, mainly OH radicals (Eqs. 6 – 11).

O2 + e-

2HO2

#### **3. Characterisation of s-triazine and phenylurea herbicides**

S-triazine herbicides contain an aromatic ring with three N heteroatoms. The formula of a triazine herbicide, atrazine, is shown in Fig. 1., the formula of a phenylurea herbicide, chlorotoluron, in Fig. 2.

Fig. 1. The structural formula of a triazine herbicide, atrazine.

Fig. 2. The structural formula of a phenylurea herbicide, chlorotoluron .

The triazine herbicides were introduced in the 1950s (Gysin & Knüsli, 1957, Gast et al., 1956, both in Tomlin, 2003), phenylurea pesticides a decade later (L´Hermite et al., 1969, in Tomlin 2003).

The solubilities of these herbicides in water are in milligrams or at most tens of milligrams per liter as shown for three trazine and one phenylurea herbicide in Table 1. Table 1 also summarizes the DT50 values for the selected herbicides. DT50 signifies 50% dissipation time, i.e. the amount of time required for 50% of the initial pesticide concentration to dissipate. Unlike half-life dissipation time does not assume a specific degradation model.


Table 1. Solubilities and DT50 values of selected triazine and phenylurea herbicides as given in Tomlin (2003).

All these herbicides are photosynthetic electron transport inhibitors at the photosystem II receptor site. They are all also systemic herbicides. Systemic herbicides (in comparison with contact herbicides) are translocated through the plant, either from foliar application down to the roots or from soil application up to the leaves. They are capable of controlling perennial plants and may be slower in action but ultimitaly more effective than contact herbicides.

A Critical View of the Photoinitiated Degradation of Herbicides 301

dealkylation to substituted aniline products. Fungal pathways result in successive dealkylated metabolites as well as aniline derivatives, but Badawi (Badawi et al., 2009) reported the detection of a new major metabolite which (according to thin layer chromatography and nuclear magnetic resonance spectrometry) is a non-aromatic diol.

Biodegradation by some bacterial and fungal strains leads to the formation of very toxic substituted anilines which have even higher levels of LD50 - the dose required to kill half the members of a tested population after a specified test duration time (Tixier et al., 2000a; Tixier et al, 2009). The same applies to products of photochemical degradation (Tixier et al.,

An organic substrate may undergo the following photoinitiated reactions under natural

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

**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

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


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

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

**5.1 Possible photoinitiated pathways for herbicide degradation** 



sunlight or artificial source irradiation: - direct sunlight photodegradation;

**5.2 Direct sunlight photodegradation** 

sunlight photodegradation.

added, no reaction occurs.

such a strong dependence on the added ferric ions.

2000b).

waters;
