Efficient activation of sodium percarbonate by hydroxylamine-enhanced zero-valent transition metals for the removal of AZO: performance, free radical generation and mechanism

Abstract

Azoxystrobin (AZO) fungicides are novel organic pollutants which are stable in water environment. These pollutants can be degraded using low cost zero-valent metals (zero-valent iron (ZVI) and zero-valent copper (ZVC)) to activate sodium percarbonate (SPC), and the introduction of hydroxylamine (HA) can effectively improve the reaction efficiency of this system. According to the experimental results, HA significantly promoted the regeneration of Fe2+ and Cu+ in the decomposed solution of SPC, significantly accelerating the formation of hydroxyl radicals (•OH) and facilitating the degradation of AZO. Under optimal conditions, when pH = 3, the rate of AZO removal by the HA/ZVI/SPC system reached 99.0 % in 15 min, while the HA/ZVC/SPC system required 50 min to reach 99.1 % removal. Both of the tested HA/zero-valent metal/SPC processes produced •OH, carbonate radicals (CO3·-), superoxide radicals (O2·-) and singlet oxygen (1O2), among which, •OH appeared to be the main reactant responsible for AZO removal. Furthermore, it was found that •OH reacted with trace levels of carbonate ions to generate other reactive oxygen species and promote AZO degradation. The addition of HA not only slowed down the surface passivation of zero-valent metals, but also promoted the conversion of high-valent metal ions to low-valent metal ions, resulting in the generation of more reactive oxygen species within the system. In addition, the degradation intermediates were identified by LC-MS analysis and density-functional theory (DFT), allowing the possible AZO degradation pathways to be proposed.