# Environmental Remediation of Metribuzin Herbicide by Mesoporous Carbon--Rich from Wheat Straw
## 3 Results and Discussion
### Adsorption Performance
#### 3.4.2 Isotherm Studies  
Isotherms are a valuable approach for assessing an adsorbent potential to remove xenobiotic compounds. Adsorption equilibrium data were simulated by current isotherm models, Freundlich and Langmuir, characteristics of which can provide essential know  
Figure 4: Influence of initial metribuzin concentration on WSB adsorption.  
Figure 5: Adsorption capacity of WS and WSB in batch experiments.  
edge about the adsorption nature and maximum adsorption extent. Experimental equilibrium results matched the Langmuir model better with R\({}^{2}\) values (0.994-0.996), which were higher than the Freundlich isotherm model values (0.84-0.97) (Figure 6 and Table 3).  
The Langmuir model indicates a single layer chemical adsorption between WS and WSB and metribuzin molecules [79]. Further, the higher value of adsorption equilibrium constant (K\({}_{\text{L}}\)) establishes strong chemical interaction between metribuzin and superficial groups of the WSB. The linearized Langmuir adsorption model is a consequence of two or more adsorption mechanisms, including external surface adsorption, diffusion into the pores of the adsorbent and chemical interactions. According to [80], the increased adsorption capacity of biochars was induced due to a larger surface area with more porous sites produced at high temperatures. A similar isotherm behavior was related for the carbofuran, carbendazim and tebuconazole pesticide adsorption on the activated carbon derived from coconut and peanut shells [3; 81].  
Table 4 presents the characteristics and reported adsorption capacities of the WSB toward metribuzin with those of other pesticides and crop straw. These results indicate that, when compared to biochars from other agri-wastes, WSB can be a suitable adsorbent material with a high adsorption potential for metribuzin molecules. Reference [14] obtained sustainable soybean biochar with a higher atrazine removal capacity. This property is assigned to the pore volume of biochar and pH. Reference [82] studied the mechanism of imidalcoprid, isoproturon and atrazine adsorption by biochar from rice and wheat straw and found a possible interaction between soil components and biochar and/or the consequence of biochar oxidation. _Pleutrotus mutilus_ based biochar was used to remove metribuzin from contaminated water, where at an acidic pH, the negatively charged active sites are protonated, thereby restricting metribuzin sorption [83]. Metribuzin sorption kinetics were designed onto electro-activated granular carbon, where the activation process speeds up three times the sorption capacity [84]. Moreover, the adsorption on WSB is  
Figure 6: Langmuir **(a)** and Freundlich **(b)** isotherms of metribuzin.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Isotherm**} & \multirow{2}{*}{**Product**} & \multicolumn{3}{c}{**Constants**} \\ \cline{3-5}  & & **q\({}_{\text{L}}\) (mg\(\cdot\)g\({}^{-1}\))** & **K\({}_{\text{L}}\) (L\(\cdot\)g\({}^{-1}\))** & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{[PERSON]} & WS & 289 & 0.46 & 0.997 \\  & WSB & 596 & 0.52 & 0.995 \\ \hline \multirow{2}{*}{[PERSON]} & WS & **K\({}_{\text{f}}\) (L\(\cdot\)g\({}^{-1}\))** & **n** & \\  & WS & 98.21 & 0.68 & 0.848 \\ \cline{1-1}  & WSB & 154.5 & 0.65 & 0.976 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Adsorption isotherm parameters for metribuzin on WS and WSB.  
advantageous toward chemical oxidation/reduction, bioremediation or the washing with extractants technique, all of which are considered expensive methods and not acceptable in large agricultural fields [85; 86].