## 3 Results and Discussion
### Possible Mechanism and Pathway of 4-CP Removal by MPHAC/PS System  
In heterogeneous systems, the mechanisms of reaction are normally more complicated than homogenous systems since the reactions are catalyst surface-limiting. In addition, the chemical reactions could occur on the heterogeneous surface of iron oxides and in the homogeneous bulk solution, and also influence the iron leaching to the bulk solution [2]. Figure 9 illustrates the possible cooperation mechanism for 4-CP removal based on the simultaneous adsorption and degradation processes in the MPHAC/PS system.  
During adsorption and degradation of 4-CP by the MPHAC/PS system, two mechanisms were involved, including (i) the adsorption 4-CP molecule by MPHAC grains, and (ii) the degradation of 4-CP molecules by the produced free activated radicals. For the  
Figure 8: Reusability of MPHAC for 4-CP removal by the MPHAC/PS system (initial 4-CP concentration: 100 mg/L; MPHAC dose: 1250 mg/L; PS dose: 350 mg/L; a solution pH of 5, and a contact time of 60 min).  
Figure 9: Simultaneous adsorption and degradation of 4-CP by the MPHAC/PS system for 4-CP removal from contaminated aqueous solutions.  
first mechanism, the 4-CP molecules are adsorbed on the surface of the MPHAC rains and removed from the solution without changing their structure. In the case of the second mechanism, the degradation process leads to the breakdown of the 4-CP molecule structure. In this case, the Fe3O4 particles present on the surface of the catalyst induce the production of iron ions, which according to Equation (1), play an important role for the enhanced activation of PS and the formation of SO4\({}^{\bullet-}\) radicals. In addition, the produced SO4\({}^{\bullet-}\) radicals can react with water molecules, according to Equation (8), and generate active \({}^{\bullet}\)OH radicals, which, together with SO4\({}^{\bullet-}\) radicals, play an important role in the 4-CP degradation [47].  
\[\text{SO}_{4}{}^{\bullet-}+\text{H}_{2}\text{O}\rightarrow\text{SO}_{4}{}^{2- }+^{\bullet}\text{OH}+\text{H}^{+} \tag{8}\]  
The produced sulfate and hydroxyl radicals react with 4-CP molecules inducing their degradation and formation of H2O and CO2 after the formation of a series of intermediate compounds, as schematized in Figure 10. As illustrated in this figure, the oxidative degradation of 4-CP leads to the formation of phenol via the reduction of 4-CP by iron ions on the surface of MPHAC [48]. In addition, due to the large amount of the generated SO4\({}^{\bullet-}\) and \({}^{\bullet}\)OH radicals, 4-CP may be attacked by the mentioned radicals during the degradation process to produce phenol and also hydroquinone [49]. This may be due to the fact that the energy of the carbon-chloride bond is lower than that of the carbon-oxygen bond, so the first broken bands are those of carbon-chloride, resulting in the release of Cl\({}^{-}\) ions [14]. In addition, SO4\({}^{\bullet-}\) and \({}^{\bullet}\)OH radicals would attack the C-C bond of benzoquinone, which was decayed to malonic acid [50]. Finally, the malonic acid was oxidized into CO2 and H2O with the action of activated radicals [48].