## 3 Results and Discussion
### Batch Desorption Studies  
#### 3.6.1 Preliminary Studies  
The preliminary adsorption study was conducted by evaluating the effect of adsorbent dosage and pH solution. The dosage effect of the adsorbents (biochar, biochar/rGO and biochar/rGO/DES-CTAB, and biochar/rGO/DES-glycerol) between the ranges of 0.001 g to 0.1 g with a constant concentration of 10 mg L\({}^{-1}\) and 50 mg L\({}^{-1}\) of ZDV and phenol, respectively, and a volume of 40 mL pollutant solution in a 50 mL centrifuge tube was investigated. The samples were wrapped with foil and left in a shaker overnight at room temperature (25 \({}^{\circ}\)C) at a speed of 200 rpm. The ZDV and phenol's percentage removal efficiency was plotted against the adsorbent dose (g), as shown in Figure 5.  
The adsorption efficiency of both ZDV and phenol increased as the adsorbent dosages increased. Adsorption peaked at 0.1 g, achieving 49% and 63% removal for ZDV and phenol, respectively (Figure 5). It was also observed that once adsorption reached equilibrium, the number of organic pollutants adsorbed remained constant as the adsorbent dosage was increased. This is due to increased particle interactions and aggregation, decreasing the total surface area of the adsorbent's active sites. Biochar showed 0% removal across all adsorbent dosages for both organic pollutants. In both experiments for the adsorption of ZDV and phenol, biochar/rGO performed better than biochar/rGO/DES-CTAB. The removal efficiency and capacity of ZDV and phenol increased as the adsorbent dosage increased (Figure 6). This is due to the increased adsorbent surface area and the vast number of available active sites for adsorption [30]. Choosing a suitable adsorbent dosage requires careful consideration. Since biochar/rGO/DES-glycerol performed better than biochar, biochar/rGO, and biochar/rGO/DES-CTAB, only biochar/rGO/DES-Glycerol will be considered for further studies and to determine the detailed effect on pH solution for the adsorption of ZDV and phenol. Due to its impact on the probable ionic formation of the adsorbate species, adsorbent surface charge, and influence of the analyte characteristics, the adsorption system's pH is a significant parameter in any adsorption study. The hydrophobic neutral adsorbate molecules could dissociate into other molecules when the pH of the adsorbate increases or decreases [31]. Increased electrostatic repulsion when the adsorbate pH is varied leads to reduced electrostatic interactions between oppositely charged adsorbate and adsorbent. An increase or decrease in pH may also positively affect the ability of the adsorbate to donate \(\pi\)-electrons, improving the \(\pi\)-\(\pi\) electron donor-acceptor interaction [32]. The effect of pH was determined with an initial concentration of 10 mg L\({}^{-1}\) and 50 mg L\({}^{-1}\) 2 DV and phenol, respectively, between pH 2.00 and 11.0 at a constant adsorbent dosage of 0.1 g biochar/rGO/DES-glycerol. The samples were left in a shaker overnight at a speed of 200 rpm. The influence of the pH solution on the adsorption efficiency showed that the maximum adsorption for ZDV and phenol occurred at pH 2.00 and gradually declined as the pH tended towards an alkaline medium. These demonstrate that acidic mediums have higher ZDV and phenol dissociation rates. Due to the activities of the functional groups on the adsorbent surface and the presence of DES, positively charged ions dominate the adsorbent surface at low pH levels. Furthermore, an increased electrostatic attraction-ion interaction mechanism between the adsorbate and adsorbent results in higher removal efficiency. This can be explained further by the adsorbent's pH (pzc). Additionally, the study of the pH effect (Figure 7) was conducted to further confirm the performance of the different adsorbents under study. The pH is a very crucial factor affecting the adsorption capacity of an adsorbent.