## 3 Results and Discussion  
### Sorption  
Among many factors affecting the transport of chemicals in soil, sorption plays an important role; therefore, sorption experiments in the presence and absence of Brij35 were carried out. Results revealed that the amount of metribuzin sorbed was higher in the absence of Brij35; however, it decreased in the presence of Brij35 at concentrations above CMC. The sorption isotherms of metribuzin in tap water and Brij35 solutions at concentrations of 0.5 and 5 g L\({}^{-1}\) (Figure 2) all showed a good fit (\(R^{2}\geq 0.97\)) with Equation (1). The \(K_{\mathrm{d}}\) value for metribuzin shows the highest 1.80 in the absence of the surfactant; however, it decreases significantly to (0.93 and 1.38) in the presence of Brij35 at 0.5 and 5 g L\({}^{-1}\), respectively (Table 2). This indicates that the presence of micelles may enhance the release of sorbed metribuzin into the solution. Furthermore, a study by [PERSON] _et al._ (1995) [41] revealed that non-ionic surfactants at concentration above CMC, decreased the sorption coefficient \(K_{\mathrm{d}}\) of hydrophobic organic compounds (HOCs). Similarly, [PERSON] _et al._ (2005) [42] indicated that Brij35 at concentration above CMC competes with atrazine for sorption sites in soil leading to a significant decrease in atrazine sorption. Furthermore, the presence of surfactants enhanced the solubility of Trichlorethylene (TEC) and therefore, higher desorption rate of TEC in the aqueous medium has been observed [43]. \begin{table}
\begin{tabular}{c c c c} \hline
**Concentration of Brij35 (g L\({}^{-1}\))** & \(K_{\mathrm{d}}\) & \(R^{2}\) & \(K_{\mathrm{ec}}\) \\ \hline
0 & 1.8 & 0.97 & 60.61 \\
0.5 & 0.93 & 0.95 & 31.31 \\
5 & 1.38 & 0.99 & 46.46 \\ \hline \end{tabular}
\end{table} 
Table 2: Distribution coefficient \(K_{\mathrm{d}}\), \(R^{2}\) and \(K_{\mathrm{oc}}\) values for metribuzin sorption in the presence of the nonionic surfactant Brij35. Figure 2: Metribuzin isotherms in the absence and presence of the non-ionic surfactant Brij35. As a result of increasing the concentration of Brij35 from 0.5 to 5 g L\({}^{-1}\), \(K_{\rm d}\) values increased from 0.93 to 1.38. This slight increase could be explained by the fact that at higher concentrations, non-ionic surfactant sorbed strongly to the soil and formed bilayers and since metribuzin has an affinity to partition with the hydrophilic head of Brij35, presumably via hydrogen bonding. This may lead to enhance metribuzin partitioning into the surfactant which is already sorbed to soil [44, 45]. Therefore, increasing the \(K_{\rm d}\) values at the highest Brij35 concentration of 5 g L\({}^{-1}\) was observed. Furthermore, [PERSON] (2005) [25] stated that the sorption coefficient of metribuzin (\(K_{\rm d}\)) was 41% lower in the presence of the nonionic surfactant degradation product nonylephenol (NP) below CMC, confirming the desorption potential of NP for metribuzin. The aqueous concentration of metribuzin increased once CMC is exceeded. This enhances micelles to form in the solution and compete with the sorbed surfactant for metribuzin. The more micelles present in the aqueous phase, the more metribuzin partitioning into the micelles within the solution, resulting in greater amount released from sorbed metribuzin. The present results are in agreement with the findings of [41, 46]. Also, our results are in good match with [PERSON] _et al._ (2009) [47] who indicated that the interaction between the surfactant and soil led to a competition with pesticides for adsorption sites, therefore a decline in pesticides adsorption was observed. The distribution coefficient \(K_{\rm oc}\), normalized to the corresponding soil organic carbon fraction \(f_{\rm oc}\) and calculated from Equation (2), is found to follow the same trend as \(K_{\rm d}\) values. Fundamentally, \(K_{\rm oc}\) values have been used as an indicator of the leaching potential of herbicides such as atrazine and metribuzin [48].