Enhanced oxidation of sulfadiazine by two-stage ultrasound assisted zero-valent iron catalyzed persulfate process: Factors and pathways

The existence of sulfonamides (SAs) in water bodies leads to a series of human health problems and environmental risks. Ultrasound (US) enhanced zero-valent iron catalyzed persulfate (ZVI/PS) process is a potential approach to eliminate SAs. However, high energy consumption and large ZVI usage limited its application. In this study, a novel two-stage US-ZVI/PS process was first proposed, which reduced 35% of ZVI usage and saved 74.2% of energy consumption compared with traditional US-ZVI/PS process. The effect of introduction time and operation mode of US on sulfadiazine (SD, 20 mg L-1) degradation was first investigated. The intermittent operation method of US (60 s on/60 s off) could help controlling the releasing of Fe2+ in a reasonable range (0.37-0.56 mg L-1), which was the key reason for the high SD degradation efficiency. SO4 center dot-, (OH)-O-center dot were confirmed as the key radicals for SD removal, and four degradation pathways were proposed. The initial pH of 5.0-7.0, the initial ZVI and PS doses of 0.6 mM and 1.4 mM were determined as the optimal conditions for high SD removal efficiency. Compared to other SD removal processes (traditional US/ZVI/PS, ozonation and UV-based process), two-stage US-ZVI/PS saved 68.3-98.5% energy consumption and enhanced 0.7-14.0 times degradation rate. Therefore, two-stage US-ZVI/PS system is promising and cost-effective for the removal of SD.