## 3 Results and Discussion
### Batch Desorption Studies
#### 3.6.2 Detailed Adsorption Studies  
The point of zero charge (PZC) is typically defined as the pH at which the net charge of the total particle surface (i.e., the adsorbent's surface) equals zero [33]. The term proposed in colloidal flocculation research explains how pH affects the phenomena. The properties of an adsorbent determine the PZC value. For example, the surface charge of an adsorbent is characterized by the ion that resides on the surface of the particle (adsorbent) structure. At a lower pH, more hydrogen ions (protons, H+) would be adsorbed than other cations (adsorbate). If, on the other hand, the surface is positively charged and the pH is raised, fewer anions will be adsorbed as the concentration of hydroxide ions rises. If the pH is less than the PZC value, the adsorbent's surface charge will be positive, allowing the anions to be absorbed. If the pH is higher than the PZC value, the surface charge will be negative, allowing cations to be adsorbed [33]. The effect of the pH solution can be explained further by the adsorbent's pH (pzc). The pH (pzc) (Figure 8) test is used to determine the pH at which the surface of the adsorbent is zero [34] and can adsorb contaminants easily. This means that the adsorbent biochar/rGO/DES-glycerol is positively charged below pH < pHpzc and negatively charged above pH < pHpzc. The adsorbent's surface charge is predominantly positively charged at pH values below their pH (pzc) [35; 36], making it more effective at absorbing anionic adsorbates at lower pH mediums. Since the pH (pzc) values are positive, it demonstrates the relevance and significance of the surface charge and solution pH during adsorption. Figure 7: Effect of pH on (**A**) 10 ppm ZDV and (**B**) 50 ppm phenol by biochar/rGO biochar/rGO/DES-CTAB and biochar/rGO/DES-glycerol. ZDV has a pka of 9.80 [37], making the ZDV molecule negatively charged below its neutral pH of 9.80 and positively charged above pH 9.80. Since the adsorbent biochar/rGO/DES-glycerol is predominantly positively charged below the pH medium of 7.40, a few mechanisms can be assumed between the negatively charged ZDV molecules and positively charged adsorbent surface in acidic media. Furthermore, ZDV is a thymidine analogue that contains a 3\({}^{\prime}\)-azide group rather than a 3\({}^{\prime}\)-hydroxyl functional group. It is believed that the 3\({}^{\prime}\)-azide group on the ZDV molecule and the surface charge of biochar/rGO/DES-glycerol in an acidic medium drives the reaction to the highest adsorption affinity of the drug achieving almost 100% removal (Figure 7). As the system's pH increased to become more basic, adsorption dropped consistently until pH 11.00, when no adsorption occurred between the ZDV molecules and the adsorbent. The adsorption of weak electrolytes, such as phenol compounds, from aqueous solutions on a porous amphoteric solid adsorbent's surface depends primarily on two crucial factors. One is the degree of electrolyte dissociation and the dominant charge on the adsorbent surface. Both factors depend on the aqueous solution's pH. The first consideration can be predicted qualitatively by the weak electrolyte's pKa (activity constant); the electrolyte will remain unaffected at a pH less than the pKa. The second factor, however, is more difficult to determine [38]. At pH = 2.00 (Figure 7), adsorbate molecules are easily combined with the positively charged surface of biochar/rGO/DES-glycerol via electrostatic attraction. Phenol has a dissolution constant (pKa) of 10.0 [39]. It ionizes more easily and has a greater polarity. As a result, phenolic molecules have increased electrostatic sensitivity. Furthermore, increasing the ionic strength of the sorbate solution appears to have a considerable influence on phenol adsorption. Unionized phenol molecules would be attracted by physical force as well. In general, phenol adsorption is most effective at lower pH values; the same trend was reported by other researchers [40; 41; 42]. There is no significant increase in the phenol's adsorption rate at pH 4.0 to 8.0.