Pretreatment of shale gas drilling flowback fluid (SGDF) by the microscale Fe0/persulfate/O3 process (mFe0/PS/O3)

Shale gas drilling flowback fluid (SGDF) generated during shale gas extraction is of great concern due to its high total dissolved solid, radioactive elements and organic matter. To remove the toxic and refractory pollutants in SGDF and improve its biodegradability, a microsacle Fe0/Persulfate/O3 process (mFe0/PS/O3) was developed to pretreat this wastewater obtained from a shale gas well in southwestern China. First, effects of mFe0 dosage, O3 flow rate, PS dosage, pH values on the treatment efficiency of mFe0/PS/O3 process were investigated through single-factor experiments. Afterward, the optimal conditions (i.e., pH = 6.7, mFe0 dosage = 6.74 g/L, PS = 16.89 mmol/L, O3 flow rate = 0.73 L/min) were obtained by using response surface methodology (RSM). Under the optimal conditions, high COD removal (75.3%) and BOD5/COD ratio (0.49) were obtained after 120 min treatment. Moreover, compared with control experiments (i.e., mFe0, O3, PS, mFe0/O3, mFe0/PS, O3/PS), mFe0/PS/O3 system exerted better performance for pollutants removal in SGDF due to strong synergistic effect between mFe0, PS and O3. In addition, the decomposition or transformation of the organic pollutants in SGDF was analyzed by using GC-MS. Finally, the reaction mechanism of the mFe0/PS/O3 process was proposed according to the analysis results of SEM-EDS and XRD. It can be concluded that high-efficient mFe0/PS/O3 process was mainly resulted from the combination effect of direct oxidation by ozone and persulfate, heterogeneous and homogeneous catalytic oxidation, Fenton-like reaction and adsorption. Therefore, mFe0/PS/O3 process was proven to be an effective method for pretreatment of SGDF prior to biological treatment.