Enhancing ferrate(VI) oxidation by ferrous iron for pretreating shale gas produced water and mitigating ultrafiltration membrane fouling

Substantial amounts of hazardous shale gas produced water (SGPW) were generated with the development of hydraulic fracturing and horizontal drilling technologies. While membrane technique has gained popularity for SGPW treatment, membrane fouling remains a significant obstacle. Herein, a novel pretreatment process of ferrate (Fe(VI)) oxidation enhanced by ferrous iron (Fe(II)) was suggested to mitigate ultrafiltration membrane fouling in the treatment of SGPW. Results indicated that Fe(II)/Fe(VI) (100/30 μM) pretreatment increased the terminal normalized flux from 0.18 to 0.47, while reducing the irreversible and reversible fouling resistances by 71.1 % and 83.6 %, respectively. Interfacial free energy assessment revealed that Fe(II)/Fe(VI) pretreatment significantly enhanced the repulsive forces between pollutants and the membrane. Furthermore, the water quality of SGPW was generally improved by Fe(II)/Fe(VI) treatment, in comparison with single Fe(VI) oxidation. The synergistic effect of Fe(II) and Fe(VI) facilitated pollutants aggregation and enhanced the conversion of organic contaminants. In general, the generated high-valent iron intermediates (Fe(IV)/Fe(V)) were key contributors to the synergistic process and prolonged the oxidation duration. Simultaneously, the formation of Fe(III) promoted the flocculation performance. This novel approach presents promise for effective SGPW treatment and membrane fouling mitigation.