## 4 Coupling Activation of Iron-Based Catalysts under Auxiliary Action
### Summary  
The coupling activation of two iron-based catalysts under auxiliary action was introduced above. Both methods can accelerate the activation effect of iron-based catalysts on activation persulfate to a certain extent.  
At present, the problem of photocatalysis is how to strengthen the application range of the photocatalyst. The treatment of industrial wastewater is generally conducted outdoors under complicated conditions. Some single photocatalysts, such as few-layered graphite-modified graphitic carbon nitride composite (GrCN), can be used in the degradation process together with PMS and photothermal catalysis [129]. The maximum reaction rate is 0.044 min\({}^{-1}\). The light source used for the data given in Table 5 is ultraviolet light, although sunlight contains only 5-7% of the ultraviolet spectrum. Therefore, how to improve the practicability of materials in the visible light range has become a new research direction.  
Furthermore, if GrCN is used alone to degrade wastewater, not to mention the effect, recycling becomes a large problem. Therefore, it is a better method to compound it with a magnetic carrier. By combining magnetic iron oxide nanoparticles with the carrier, not only can the excellent catalytic effect of iron oxide be brought into play, but it can also facilitate the recovery of GrCN as a carrier, so that the two substances complement each other [130].  
Piezoelectric catalysis, as a newly developed technology in recent years, still has a great space for development. Through ultrasonic and other methods that can generate vibration, oxidants and catalysts can be evenly dispersed into sewage to increase the contact point of chemical reaction, and more electrons can be generated by promoting piezoelectric materials to increase the concentration of free radicals and accelerate the progress of chemical reaction.  
Table 7 shows the advantages and disadvantages of representative five iron-base catalysts mentioned above.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Catalyst** & **Advantages** & **Disadvantages** & **Ref.** \\ \hline CoFe\({}_{2}\)O\({}_{4}\) & \begin{tabular}{c} CoFe\({}_{2}\)O\({}_{4}\) exhibited an \\ excellent performance for \\ ATZ removal (over 99\%). \\ \end{tabular} & \begin{tabular}{c} It has a good effect in activating PMS, \\ but in activating PS and H\({}_{2}\)O\({}_{2}\); its \\ recycling rate is not good due to the \\ leaching of metal ions and loss of \\ active sites. \\ \end{tabular} & [53] \\ CuFe\({}_{2}\)O\({}_{4}\)- & \begin{tabular}{c} Increase of specific surface \\ area and chemical reaction \\ activation sites. \\ \end{tabular} & \begin{tabular}{c} The most suitable pH is 6.5; the \\ application is limited. \\ \end{tabular} & [64] \\ Fe\({}^{0}\)@Fe\({}_{3}\)O\({}_{4}\) & \begin{tabular}{c} The synthetic route is complex. It has a \\ low stoichiometric efficiency (10.3\%) \\ because most PMS are ineffectively \\ consumed during activation. \\ \end{tabular} & [90] \\ US/ Fe\({}^{0}\) & \
\begin{tabular}{c} It is ambicked pH range is low (under 4). \\ Too much catalyst will also reduce the \\ reaction rate, so the use of catalyst needs \\ to be strictly controlled. \\ \end{tabular} & [110] \\ US/ Fe\({}^{0}\) &
\begin{tabular}{c} It is easily affected by the action of other \\ anions in the solution (Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\)). \\ \end{tabular} & [125] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Advantages and disadvantages of representative five iron–base catalysts mentioned above.