Selective oxidative degradation of toluene for the recovery of surfactant by an electro/Fe2+/persulfate process

An electro/Fe2+/persulfate process has been conducted for toluene removal from surfactant (SDS) flushing solution, and the pseudo-second-order reaction rate constant (k (2) value) of toluene removal has been optimized by a response surface methodology (RSM). The results indicated that in this process, the reaction between persulfate and externally added Fe2+ generates sulfate-free radicals, and at the same time, Fe2+ is electro-regenerated at the cathode by the reduction of Fe3+. RSM based on Box-Behnken design (BBD) has been applied to analyze the experimental variables, of which the concentrations of persulfate and Fe2+ showed a positive effect on the rate constant of toluene removal, whereas the concentration of SDS showed a negative effect. The interactions between pairs of variables proved to be significant, such as between SDS, persulfate, and Fe2+ concentrations. ANOVA results confirmed that the proposed models were accurate and reliable for analysis of the variables of the electro/Fe2+/persulfate process. The shapes of the 3D response surfaces and contour plots showed that the SDS, persulfate, and Fe2+ concentrations substantially affected the k (2) value of toluene removal. The results indicated that increasing persulfate or Fe2+ concentration increased the k (2) value, whereas increasing SDS concentration decreased the k (2) value. The reaction intermediates have been identified by GC-MS, and a plausible degradation pathway for toluene degradation is proposed.