## 3 Results and Discussion
### Adsorption Isotherms  
Isothermal adsorption models are fundamental tools in evaluating and optimizing adsorption processes between adsorbates and adsorbents. These models allow researchers to predict the behavior of the adsorption system under different conditions and provide insights into the adsorption capacity, affinity, and mechanisms. In this study, the Freundlich isotherm model was applied to describe the adsorption of 4-MBC on biochar derived from corn, hemp, and straw. As shown in Figure 5 and detailed in Table 4, the adsorption parameters obtained from the Freundlich model are essential for understanding how different biochars interact with 4-MBC, with a particular focus on their adsorption capacity and surface characteristics. The high correlation coefficients (\(R^{2}>0.9\)) across all biochars confirm the suitability of the Freundlich model in describing the adsorption of 4-MBC on these materials. The Freundlich exponent, N, provides insight into the nature of the adsorption process, with values deviating from unity (N \(\
eq\) 1) suggesting nonlinear behavior. In this case, the nonlinear adsorption indicated by N values reflects the complexity of the adsorption process, involving multiple mechanisms such as surface adsorption, pore filling, and interaction with the functional groups present on biochar surfaces. This nonlinearity points to the heterogeneity of the adsorption sites, which could be linked to variations in the biochar's surface chemistry and morphology. The observed higher adsorption coefficient (\(K_{\text{F}}\)) for biochar derived from corn, compared to other feedstocks, is likely due to its higher specific surface area (SSA). A larger SSA provides more adsorption sites for 4-MBC, enhancing the adsorptive interaction between the adsorbent (biochar) and adsorbate (4-MBC). Numerous studies have established a positive correlation between SSA and adsorption capacity, particularly for biochars and activated carbons, as the increased surface area provides more available sites for physical and chemical interactions between organic pollutants and the adsorbent. This underscores the importance of optimizing biochar production parameters, such as pyrolysis temperature, to maximize SSA and improve adsorption efficiency in environmental applications, including wastewater treatment and pollution remediation [56; 57; 58; 59]. In addition to the biochar experiments, the study also employed multiple isothermal adsorption models to better understand the adsorption of 4-MBC on MPs. The isotherms and adsorption parameters for the MPs (PEg, PP, PET, PLA) are summarized in Table 5. Figure 6 presents the fitted curves for the Freundlich and Langmuir models, which were used to characterize the adsorption behavior. The correlation coefficients for the Langmuir model ranged from 0.778 to 0.980, with the best fit for PEg, PP, and PET. This suggests that for these MPs, the adsorption of 4-MBC occurs at specific sites, following the Langmuirassumption of monolayer adsorption. The higher qmax values for PET (16.6 \(\upmu\)g/g) compared to PEg (9.36 \(\upmu\)g/g) reflect the influence of polymer properties, such as hydrophobicity and functional group availability, on adsorption capacity. Interestingly, the Freundlich model provided a better fit for the adsorption of 4-MBC on the biodegradable plastic PLA, which exhibits a more heterogeneous surface due to the presence of functional groups capable of interacting with water molecules. This leads to the formation of water clusters, affecting the overall adsorption process and suggesting multilayer adsorption behavior. The n values for PLA (ranging between 0.15 and 0.50)