Performances and mechanisms of ferrate(VI) oxidation process for shale gas flowback water treatment

Abstract

The aim of this study was to determine effective and green methods for the removal of organic matter from shale gas flowback water (SGFW). The effects of ferrate (Fe(VI)), initial pH, and reaction time on the removal efficiency of Fe(VI) were investigated. A chemical oxygen demand removal efficiency of 57 % was achieved under the optimal conditions of Fe(VI)= 1500 mg/L, pH= 8.0, T = 25 °C, 60 min. Combined with the results of the chemical probe method, competitive kinetic experiments, electron spin resonance spectra, and radical quenching studies, hydroxyl radicals (∙OH) were demonstrated as the dominant reactive species responsible for oxidation. Furthermore, the fluorescence intensity of soluble microbial by-product-like matter and acid-like components were found to be reduced by 64 % and 43 %, respectively, using fluorescence excitation-emission matrix spectroscopy tests. The results of gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry indicated that electron-rich organic compounds were easily removed by Fe(VI) oxidation. Bacterial culture and coagulation experiments showed that Fe(VI) had both disinfection and coagulation effects in the SGFW. Furthermore, considering H2O2 as a precursor for ∙OH generation, the combined H2O2-Fe(VI) process was investigated in this study, and the results showed that the process has a synergistic effect and can increase the Chemical Oxygen Demand (COD) removal up to 68 % (Fe=1500 mg/L, pH=8.0, H2O2 =0.05 mol/L, T = 25 °C). Finally, an ultrafiltration membrane fouling experiment proved that the oxidation processes can increase the membrane-specific flux and alleviate fouling resistance. This study provides a reference for the design and operation of organic removal process in the SGFW treatment.