Efficient removal of organic contaminants in real shale gas flowback water using Fenton oxidation assisted by UV irradiation: Feasibility study and process optimization

•High TOC removal and synergistic effects were achieved using UV-Fenton process.•Operating conditions of the UV-Fenton process were optimized using response surface methodology.•TOC removal by the UV-Fenton process can be explained by second-order kinetics.•A possible degradation mechanism of dibutyl phthalate was proposed. [Display omitted] The aim of this study was to identify an effective method for removing organic pollutants from real shale gas flowback water (SGFW). The results showed that ozonation could only achieve limited total organic carbon (TOC) removal when the initial pH was set to 3 or 7. Compared with the O3/H2O2 and Fenton processes, better performance was observed with the ultra violet (UV)-Fenton process in SGFW. The effects of H2O2/COD, initial pH, H2O2/Fe2+, and reaction time on the removal efficiency via the UV-Fenton process were investigated through single-factor experiments. The optimal experimental conditions were obtained using a quadratic polynomial prediction model (RAdj2 = 0.9295) from the central composite design of response surface methodology. The TOC removal efficiency could reach as high as 70.02% even in real SGFW under optimal conditions (H2O2/chemical oxygen demand ratio 11.54, H2O2/Fe2+ ratio 130.20, pH 3.72, temperature 25 °C, and reaction time 60 min). Moreover, the TOC removal kinetics could be better explained by pseudo-second-order kinetics (R2=0.9609). In UV-assisted Fenton oxidation, the cycle of Fe3+ species to Fe2+ was accelerated to regenerate more ∙OH radicals, thereby leading to higher TOC removal efficiency. Moreover, gas chromatography-mass spectrometry analysis revealed that the UV-Fenton process could mineralize most organic pollutants in the SGFW. Dibutyl phthalate was selected as the model organic matter to explore the possible degradation mechanism. This study provides a reference for the design and operation of organic removal units in the SGFW treatment process.