## 2 Source and Modification Methods of Biochar for Pesticide Remediation
### Modification Methods of Biochar  
Biochar can be modified by gas activation, ball milling, radiation, acid, alkali, oxidant, metal ion, and other treatment methods illustrated in Figure 2. For instance, modified biochar with increased surface area, porosity, and/or functional groups of a material in order to increase its sorption capacity has garnered more attention in recent years [22]. The choice of method depends on its application field [71]. The modification of biochar can significantly improve its activity and increase its application potential in pesticide-polluted environmental remediation [72]. Modification of biochar with chemical reagents may change its physicochemical characteristics, thereby improving its sorption capability, as found in Table 2.  
As shown in Table 2, the biochar modification methods include acid, alkali, oxidant, metal ion, and others. Compared with that of unmodified biochar, the maximum adsorption capacity of modified biochar ranging from 2.84 to 269.4 mg \(\cdot\)g\({}^{-1}\) was significantly improved by increasing the adsorption sites and promoting the function of biochar. The removal rates varied from 87.52% to 99%, which were mainly focused on herbicides and insecticides, while there is little research on fungicides. The sorption capacity and removal rate increased, suggesting that high pH is favorable for sorption on biochar. Pesticide adsorption kinetics can be well-represented by both the pseudo-first and pseudo-second order nonlinear kinetic models. In a word, modified biochar provides better remediation effects on pesticide-contaminated soils. Using different biochar and modification methods had led to differences in the adsorption mechanism of pesticides. When most of the biochar is modified by chemicals, the pore filling mechanism is lower, increasing chemical adsorption. Most mechanisms are \(\pi\)-\(\pi\) interactions and hydrogen bonds. Biochar prepared from rice husk has more, also  
Figure 2: Common modification methods of biochar. involving soil, seeds, and plants. Biochar made from rice bran, rice husk, reed powder, and corn straw has more than four adsorption mechanisms for pesticides, while cassava, slurry, and pine needle have only one adsorption mechanism for pesticides. In particular, the effect of modified biochar on the environmental behavior of maximum pesticide adsorption capacity (mg\(\cdot\)g\({}^{-1}\)) and removal rate (%) is to be considered. The modification methods of biochar are mainly chemical methods, and pH before and after modification is related to the added modified substances (Table S2). After the modification of the biochar with acid substances, the pH of the modified biochar decreases, and the range is about 1.45 to 1.89. Under the same modification conditions, due to different raw materials prepared, the properties after modification are generally different. By modification, biochar's environmental behavior of maximum pesticides adsorption capacity (mg\(\cdot\)g\({}^{-1}\)) and removal rate (%) is increased (Table S2). The specific surface area (mg\({}^{2}\cdot\)g\({}^{-1}\)) and total pore volume (cm\({}^{3}\cdot\)g\({}^{-1}\)) of the modified biochar are mostly increased. The removal rate of pesticide pollutants from corn straw biochar modified with acid substances increases from 38% to 96%. The maximum adsorption capacity of oil palm empty fruit modified by chitosan increases from 8% to 75%. Due to different modification methods for biochar made of the same raw materials, the maximum adsorption capacity and removal rate of the modified biochar for the same pollutant are improved. The maximum adsorption capacity of atrazine by modified corn straw biochar is about 47.22 to 48.09 mg\(\cdot\)g\({}^{-1}\), and the maximum removal rate is 23.61% to 58%. As shown in Table S2, experimental result is greatly related to the influence of raw materials.