Perfluorooctanoic acid (PFOA) is a persistent organic pollutant. PFOA is a toxicant in animals and enhances the health risks. It is difficult to remove PFOA using most conventional wastewater treatment techniques. Recently, the development of PFOA degradation techniques is one of the current research hotspots. Photodegradation is one of the most promising alternatives to decompose PFOA efficiently. TiO2 photocatalysis is widely used in the environmental fields due to its stability and low cost. Graphene is a two-dimensional nanomaterial and has great potential in various applications, due to its excellent features in a high electrical and thermal conductivity, high mechanical strength, high carrier mobility, high transparency and huge specific surface area. The nanocomposites of TiO2 and graphene exhibit high photocatalytic activity. Although PFOA degradation technology has made great process, there are also some thorny problems, such as the poor efficiency of PFOA direct photolysis, the controversial mechanisms of PFOA photocatalysis by TiO2 and the lack of investigate for the PFOA photocatalysis by the nanocomposites of TiO2 and graphene.
      In this study, we prepared the nanocomposites of TiO2 and graphene (TiG); explored the relationship between the structure of TiG and photocatalysis activity; investigated the temperature effect on PFOA direct photolysis; tried to degrade PFOA using TiG as photocatalyst and speculated the photocatalysis mechanism. The specific results are as follows:
      (1) The structure of TiG nanocomposites. The TiO2 component in TiG is anatase phase. TiO2 nanoparticles with a narrow size distribution disperse on the surface of graphene sheets uniformly when the graphene content increase. The graphene component increase the surface area of TiG nanocomposites greatly. The TiO2 is bond on the site of oxygen functional groups in graphene with Ti-O-C bond. The Ti-O-C bond modifies the near edge structure of Ti element in pure TiO2. In a word, TiG has high surface area and load anatase TiO2 grown on graphene with Ti-O-C bond.
      (2) The photocatalysis performance of different TiG nanomaterials. TiG20 (graphene content 20 wt. %) can completely decolor Rhodamine 6G in 20 min. The apparent rate constant of TiG20 is 6.3 times than that of TiG1. The aromatic structures of TiG enhance the π-π stack adsorption of Rhodamine 6G and accept photo-induced electron. The Ti-O-C bond is favorable for the transfer of photo-induced electron. The aromatic structures and Ti-O-C bonds inhibit the recombination of photo-induced electron-hole pair and thus promote the photodegradation. TiG20 has an optimal dosage (4.0 mg·L-1). The photodegradation is not enough to degrade Rhodamine 6G when the dosage is small. The degradation efficiency decreases when the dosage is big due to the light scattering and reflecting. In a word, the increase of graphene content in TiG nanocomposites enhances the photocatalysis activity.         
      (3) Temperature effect on PFOA photolysis. The high temperature enhances the decomposition of PFOA. At 85 °C, the direct photolysis could almost completely remove PFOA in 60 min. The quantum yield at 85 °C (1.72 × 10-3) is 12 times higher than that at 25 °C (1.55 × 10-4). The high temperature promotes each cyclical process of losing one CF2 unit and thus enhance the production of fluorine ions. At 85 °C, the defluorination ratio achieve 72% in 180 min. The energy consumption for removing one μmol PFOA reduced from 82.5 kJ at 25 °C to 10.9 kJ at 85 °C. The high temperature enhances PFOA photolysis and defluorination using low energy consumption.
      (4) PFOA photocatalysis by TiG nanocomposites. At 25 °C, the rate constant in direct photolysis is 0.693 × 10-2 min-1, the rate constant in TiG20 degrading system increase to 1.406 × 10-2 min-1. At 60 °C, the rate constant in direct photolysis was 4.373 × 10-2 min-1, the rate constant in TiG20 degrading system decreased to 0.361 × 10-2 min-1. In TiG photocatalysis system, there are  two different  kinds of  PFOA degradation min  ,  the rate  constant in  TiG20 degrading system processes, including  direct photolysis and  β-scission. At 25  °C, β-scission accounted for a large proportion of the  PFOA degradation. At 60 °C, direct photolysis accounted for a larger proportion. TiG20 nanocomposite is prone to produce •OH at 60 °C, which is not effective to  degrade PFOA. TiG20 nanocomposite enhances  PFOA degradation at 25 °C but inhibits PFOA degradation at 60 °C.
      We realized the efficiency enhancement of PFOA direct photolysis and investigated PFOA photocatalysis by the nanocomposites of  TiO2 and graphene (TiG). Both direct photolysis and β-scission  existed in TiG photocatalysis system.  At room temperature, TiG promoted the degradation  of PFOA through β-scission chemical process. At  high
temperature, TiG20 nanocomposite  was prone to  produce •OH and thus  inhibited the degradation of PFOA.