Halogenated organic contaminants (HOCs), including perfluorinated or partially fluorinated chemicals (PFCs), chlorinated organic compounds (COCs) and brominated organic compounds (BOCs), show the characteristics of persistent organic pollutants after they are discharged into the environment. Optimized photochemical elimination methods, which involve the pollutant properties, light adsorption and quantum yield, interfacial interaction and environmental conditions, are promising techniques for controlling the potential risk of HOCs. The present review summarizes the mechanisms of photocatalytic degradation of perfluorooctanoic acid (PFOA), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) by photo-generated radical species (eg, h_VB⁺, e_CB^-, ·OH, e_aq^-) on photocatalyst as well as the mechanisms of direct photolysis. The photochemical degradation of HOCs can be achieved through the electron transfer between HOCs and reactive radical species or photo-induced reduction from the excited state of HOCs. Optimazation designs of photocatalyst and reaction process are important factors for the photocatalytic degradation of HOCs. Based on the currently achieved research results, some key issuses for the future are proposed from some aspects such as the interfacial electron transfer, the information of band structure, and the integration of reaction and separation.

Contents
1 Introduction
2 The mechanisms of photochemical degradation of HOCs
2.1 The photocatalytic degradation of HOCs
2.2 The direct photolysis of HOCs
3 Influencing factors on photocatalytic degradation of HOCs
3.1 Effect of photocatalytic materials
3.2 Effect of reaction condition
4 Conclusions