According to the reaction of Fe2+ and H2O2 for OH generation, the effect of H2O2 concentration on the degradation of organic pollutants by various Fenton optimization processes is similar to that of Fe2+ concentration. Therefore, H2O2 is also a highly important parameter affecting the degradation efficiency of organic pollutants. Usually the degradation efficiency of organic pollutants increases with the increasing of H2O2 concentration (Rivas et al., 2001; Li et al., 2018). However, H2O2 cannot be added without any limitation. The excessive H2O2 not only increases the operational costs, but also enhances the scavenging effect of OH by H2O2 (Eq. (10)) (Borràs et al., 2010; Hu et al., 2018), which has a negative effect on the degradation of organic pollutants. In order to obtain the maximum removal of organic pollutants, the optimal H2O2 concentration should be examined. For example, the work of Ramirez et al. (2007) showed the optimal degradation efficiency of orange II could be obtained at 6 mM of H2O2 in the heterogeneous Fenton process, while further increasing of the H2O2 concentration to 48 mM does not obtain an increase but decrease in degradation efficiency.