The role and fate of EDTA in ultrasound-enhanced zero-valent iron/air system

In this study, factors such as ZVI loading, ultrasound (US) input power, reaction temperature and solution pH that could affect the EDTA degradation in the US-enhanced zero-valent iron/air (US/ZVI/air) system were investigated. In the system, EDTA also served as an agent for complexation with the dissolved Fe2+ and producing H2O2 that would trigger Fenton-like reaction and degrade the EDTA itself. US played a threefold role in enhancing the overall EDTA degradation rate through: (1) accelerating ZVI corrosion and production of Fe2+; (2) enhancing H2O2 production through overcoming the kinetic barrier of oxygen activation by iron-EDTA complex; (3) improving EDTA mineralization while reducing its harmful by-products. The EDTA degradation rate with the US/ZVI/air system was 7.8 times higher than the combined rates achieved with US/air and ZVI/air operated as individual systems, indicating a significant synergistic effect. A hypothetical scheme illustrating the role and fate of EDTA in the US/ZVI/air system is proposed, indicating a complex auto-oxidation process enhanced by the US. It is postulated that ferryl-EDTA complex ([FeIVO]EDTA) rather than OH was the dominant oxidant generated in the US/ZVI/air system that oxidized the EDTA even under the circumneutral condition.