## 3 Results and Discussion
### Factors Affecting the Removal of 4-CP by the Combined MPHAC/PS System  
#### 3.3.1 Effect of the Initial Solution pH on 4-CP Adsorption and Degradation  
One of the most important and fundamental factors in advanced oxidation processes is the pH of the solution to be treated, because of its importance in the ionization of pollutants and oxidants, pollutant decomposition, catalyst surface structure, oxidant activity, and solubility [37]. For instance, it plays an important role on the activity of active species in the MPHAC/PS system, as shown in Figure 4, presenting both the adsorption and degradation of 4-CP at different pH values 3, 5, 7 and 9. Figure 4: Effect of the initial solution pH on 4-CP removal factor using the MPHAC/PS system (initial 4-CP concentration: 100 mg/L; MPHAC dose: 150 mg/L; and PS dose: 150 mg/L). Figure 3: 4-CP removal factor of MPHAC, PS, and the combined MPHAC/PS system as a function of the reaction contact time (4-CP concentration: 150 mg/L; MPHAC dose: 250 mg/L; PS dose: 150 mg/L; and a solution pH of 7). As can be seen from Figure 4, in acidic conditions, the removal of 4-CP was higher than in alkaline conditions. The percentage of 4-CP removal decreased dramatically from 80 to 50%, as the initial pH increased from 5 to 9 due to the electrostatic attraction between the 4-CP molecules and the surface of MPHAC. It is necessary to know the pH\({}_{\text{PZC}}\) of the catalyst and pK\(a\) of 4-CP in order to analyze the observed different 4-CP removal factors of the MPHAC/PS system at different pH values. The obtained pH\({}_{\text{PZC}}\) value of the synthesized MPHAC in this study was 6.35. When the pH of the solution is higher than 6.35 (pH > pH\({}_{\text{PZC}}\)), the surface charge of the catalyst becomes negative, and at pH values below 6.35 (pH < pH\({}_{\text{PZC}}\)), the surface load of the catalyst becomes positive [28]. On the other hand, taking into consideration that the 4-CP pK\(a\) value is 4.7, 4-CP has a positive charge in acidic conditions (pH < pK\(a\)), preventing the adsorption of 4-CP on the surface of the catalyst. According to the above description, the highest removal efficiency was observed when pK\(a\) < pH < pH\({}_{\text{PZC}}\). In other words, in this pH range, 4-CP molecules have an anion form and are adsorbed on the surface of the catalyst having a positive charge. Moreover, the observed higher 4-CP removal factors in acidic solutions compared to alkaline solutions is attributed partly to the fact that in alkaline conditions, PS is repelled from the catalyst surface due to its anionic property, released in the solution, and its activation is stopped as consequence [38]. This phenomenon is noticeable at pH 9, confirming the decrease of the PS activity. The obtained results are in line with those reported by [PERSON] et al. [39], who studied propachlor degradation by ferrous and copper ion activated PS, and reported that at basic pH conditions, the activation of PS decreased via copper hydrolysis. However, the findings were opposite to those of [PERSON] et al. [40], who demonstrated that the combination of biochar and PS had relatively good activation efficacy of PS under a wide range of initial pH 4-9 values. In addition to the effect of the initial pH value on the 4-CP adsorption and degradation processes of the MPHAC/PS system, the effect of the reaction time on the 4-CP removal process was investigated in order to understand the behavior of 4-CP adsorption and degradation along the contact reaction time. As shown in Figure 4, in all ranges of pH values, at the beginning of the reaction, the adsorption process dominated the 4-CP removal process. Then, over time, the degradation process becomes gradually dominant for all test pH values, except at the pH 9, in which the adsorption process is maintained to be higher than the degradation process. At the beginning of the reaction, PS does not have enough time to be activated and 4-CP removal often occurs by the adsorption process.