We used several types of water samples (distilled water, well water (h = 0.5 m), run-off water, well water (h = 1 m) and river water). For each sample, we determined the amount of dissolved organic matter and the initial pH. To determine the total organic carbon (TOC) concentrations, we first determined the total carbon concentrations (TC) and then the inorganic carbon concentrations (IC). The difference (TC - IC) gives us the TOC. The experiment shows that the values found vary with the sample. All the results are shown in Table 2 .

TC = total carbon; IC = inorganic carbon; TOC = total organic carbon (TC - IC); pH_i = pH of natural water.

This table shows that dissolved organic matter concentrations vary with the matrix independently of pH. TOC values are relatively higher in river water and well water 1 m deep, followed by run-off water and well water (0.5 m deep) and finally distilled water. Photodegradation reactions will be more or less rapid in environments with higher concentrations of dissolved organic matter. This is because dissolved organic matter can accelerate or delay photodegradation reactions.

Figure 1 shows the electronic absorption spectra of haloxyfop-methyl in samples of well water (0.5 m) and river water over the irradiation period. The spectrum of this non-irradiated product shows a band located between 325 and 525 nm, with a peak at around 380 nm. The same result was obtained in the other types of water, showing that haloxyfop-methyl absorbs in the same zone whatever the aqueous matrix.

As irradiation progresses, a decrease in peak intensity is observed in all the cases studied, indicating that haloxyfop-methyl is unstable under the effect of solar radiation in natural waters. This result is in good agreement with the literature. Indeed, studies have shown that photodegradation of haloxyfop-methyl (esters) rapidly gives different photoproducts such as haloxyfop-R (acid), pyridinol, pyridinone and phenol [6] [10] .

To better assess the impact of solar radiation on the degradation of haloxyfop-methyl in the different aqueous matrices, we determined the percentage of photodegradation of this herbicide in each aqueous matrix and the half-life times.

Figures 2(a)-(b) show the curves $\ln \frac{A_0}{A}=f\left(t\right)$, respectively for well water (depth 0.5 m) and river water. We obtained lines with a positive slope and a correlation coefficient close to unity, indicating kinetics of order 1. The same result was obtained in all the other cases studied.

From these lines, we determined the rate constants and deduced the half-lives, using the following relationships: $\ln \frac{A_0}{A}=kt$, $t_{1/2}=\frac{\ln 2}{k}$.

Where the rate constant k corresponds to the slope of the straight line.

To better assess the effect of solar radiation on our product in the various matrices studied, we also determined the percentage of photodegradation for a duration of 60 minutes using the formula: $\left(\frac{A_0-A}{A_0}\right)\times 100$.

In this calculation, A represents the absorbance at an irradiation time equal to 60 minutes (tirr = 60 min) and A0 represents the absorbance at tirr = 0 min.

All the results for the photodegradation kinetics of our product are shown in Table 3 .

k = photodegradation rate constant (min⁻¹); t_1/2 = half-life of the photodegradation reaction (min); PP = Percentage of photodegradation; r² = correlation coefficient.

This table shows that the photodegradation of our product obeys first-order kinetics whatever the matrix studied. The same result was obtained by Khonté et al. [12] when they studied the photodegradation of the herbicide rimsulfuron in natural waters. Similar kinetics were also found by D.D. Thiaré et al. [13] when they studied the photodegradation of the fungicide Benomyl in distilled water, seawater, well water and river water. Similar kinetics were also found by D. Sarr et al. [14] when they studied the photodegradation of the herbicide fluometuron in water using UV and fluorescence. This shows that the photodegradation of pesticides in aqueous media generally follows kinetics of order 1.

This table also shows that the photodegradation rate constants for haloxyfop-methyl are relatively high, corresponding to relatively low half-lives. The half-lives found range from 9.18 to 47.47 minutes. This result shows that haloxyfop-methyl is very sensitive to light in natural waters, compared with other pesticides subjected to the action of solar radiation or other more powerful sources of energy, such as ultraviolet radiation. In studying the photodegradation of the herbicide rimsulfuron under the effect of solar radiation in natural waters, Khonté et al. [12] found half-lives in excess of 144 minutes, even in the presence of a photosensitiser. In another study conducted on the degradation of two endosulfan isomers (a-endosulfan and b-endosulfan) under the effect of solar radiation in borehole water, S. K. Traoré et al. [15] observed a certain stability. The half-lives found were 770 hours and 407 hours respectively. Other authors such as D. Sarr et al. [16] and D. D. Thiaré et al. [13] and Med Lemine Ould et al. [17] , studying respectively the degradation of fluometuron, benomyl and metalaxyl by UV radiation in aqueous media, found half-lives 35 times higher for fluometuron, between 2.75 and 26. 85 times higher in the case of benomyl irradiated in various natural waters and 7.6 times higher in the case of metalaxyl compared with haloxyfop-methyl when the latter was irradiated under solar radiation in river water, where its degradation was slower. This shows that haloxyfop-methyl is highly unstable under the effect of solar radiation in natural waters. However, the quantity transformed varies according to the matrix studied. It is relatively higher in distilled water, but lower in river water. In all the cases studied, we note that the percentage of photodegradation of this product is greater than that obtained by S. Halladja et al. [18] , when they studied the photodegradation of fluometuron under the effect of solar radiation in water. The results of their study showed that less than 30% of this product had disappeared after 6.7 days. A correlation was observed between the rate of disappearance of this product and the quantity of dissolved organic matter. In distilled water, where the quantity of organic matter was lower, degradation was faster. On the other hand, in river water, where the quantity of organic matter was higher, degradation was slower. The slow photodegradation observed in river water could therefore be attributed to the high presence of dissolved organic matter. This phenomenon could be explained, as in the case of the photolysis of Carbofuran [19] , Diuron [17] and metalaxyl [20] in the presence of dissolved organic matter, by the fact that the dissolved organic matter fixes the molecules of the product and absorbs much of the UV radiation. This could slow down the photodegradation reaction. This result obtained in the case of our product is different from that obtained by Regi Mathew and colleagues [21] when they studied the photodegradation of the herbicide metolachlor in water in the presence of dissolved organic matter. In this study, the rapid photodegradation of this herbicide in the presence of dissolved organic matter could be explained by the presence of free radicals produced by the dissolved organic matter, which would be capable of initiating the photodegradation reaction of the herbicide molecules. This difference in behaviour noted in the two studies shows that dissolved organic matter plays a dual role:

Two 2 m² grass beds were used to assess this herbicidal activity.

Using the protocol described in 2.3.3, we observed yellowing of the grasses exposed to the sun from the sixth day after application of the haloxyfop-methyl solution ( Figure 3 ). It was not until the sixteenth day that the grass was completely dried out ( Figures 3(B)-(D) ). In the case of the plane kept away from light, the herbicidal action was more obvious. In fact, the majority of grasses were dried out in less than four days and total drying was observed from the sixth day of application of our product ( Figure 3(B) ). These results show that the herbicidal activity can be attributed to the product or its photoproduct (haloxyfop-R), which also

has her bicidal activity ( [6] ). However, the herbicidal activity observed was greater without irradiation. This activity could therefore come mainly from the initial product. Several authors, including Buhler et al. [22] and Poiger et al. [23] , have demonstrated that haloxyfop-methyl has greater herbicidal activity and greater efficacy than certain herbicides in the same family, such as fluozifop-buthyl or its metabolite haloxyfop-R. The results of our study are therefore consistent with these observations. These same authors also showed that the herbicidal activity of a compound or its degradation product can depend not only on the nature of the plant studied, but also on the sources of degradation, which can lead to the formation of different degradation products.