## 5 Conclusions and Prospect  
Activation persulfate technology based on iron-based catalysts has attracted wide attention in recent years. As for iron-based catalysts themselves because the central atom is Fe, their electron configuration is not in a full or partially full state, so their chemical properties are relatively active, and they are susceptible to not only pH but also various ions in water. SR-AOPs-PMS/PDS technology is new, and several metal ions mentioned above can activate persulfate. In terms of the current problems, several ideas and possibilities for improvement are proposed:  
Adopt the multimetal composite method in the advanced oxidation technology of bimetal coordination. Taking iron as the core, screen and compare other metal ions of transition elements, and compound new iron-based catalyst.  
Based on the data above, it is not difficult to see that the AOPs technology of nonfree radicals also has ideal effects. Compared with the generation of \(\cdot\)SO\({}_{4}\)\({}^{-}\), the degradation effect of \({}^{1}\)O\({}_{2}\) produced by nonfree radicals is better to find a new nonfree radical reaction pathway to degrade wastewater.  
After the reaction stops, the solubility of some iron-based catalysts or metal leaching may produce iron slag and other wastes. If the subsequent treatment is improper, it is easy to cause secondary pollution to the water body; at the same time, there is metal valency reaction in the reaction process, which reduces the collision of effective molecules and has more side reactions. How to improve the effective ion concentration for the reaction has become an urgent problem to be solved.  
The persulfate used in the reaction is strongly oxidizing. If stored improperly or used in excess, it will produce a large toxic effect on organisms.  
Most of the iron-based catalyst reaction conditions are acidic and create a strong acid environment. It is not applicable in a neutral environment. If used for degraded wastewater, the pH of water needs to be adjusted, and if there is acid intolerance or acid decomposition substances in the water, then it is easy to produce adverse consequences. Furthermore, the pH of water should be adjusted after the reaction is terminated, which greatly increases the cost.  
Coupled catalysis based on iron-based catalysts with auxiliary action has played a good role in activation persulfate, and it is worthy of further study to improve the degradation effect by applying additional conditions. We can try to improve the performance of the materials with poor degradation effect among the 12 different iron-based catalysts mentioned above by photocatalysis, piezoelectric catalysis, and other methods.  
Piezoelectric catalysis, as a research hotspot in recent years, is in its infancy at present. More kinds of catalysts can be produced by attempting to combine other transition metals and oxides with iron-based materials.  
Conceptualization, K.Z. and Z.L.; methodology, P.M.; software, Z.L. and P.M.; validation, Z.L., P.M., W.Z. and Y.T.; formal analysis, K.Z.; investigation, Y.Z. and W.Z.; resources, K.Z.; data curation, P.M.; writing--original draft preparation, Z.L. and J.Z.; writing--review and editing, Z.L.; visualization, Z.L.; supervision, K.Z.; project administration, K.Z.; funding acquisition, K.Z. All authors have read and agreed to the published version of the manuscript.  
This review was funded by the Research Foundation of China University of Petroleum-Beijing at Karamay (grant number: YJ2018B02002). This work was financially support by the Research Foundation of China University of Petroleum-Beijing at Karamay (grant number: YJ2018B02002). Thanks to the reviewers and editors for their valuable comments. I would like to express my heartfelt thanks to every author of the research group who actively participated in the work.  
The authors declare no conflict of interest.