Hydroxylamine-assisted retarding surface passive layer of zero-valent iron for highly efficient p-nitrophenol removal via direct electron transfer

Zerovalent iron (ZVI) has been widely used in groundwater remediation due to its excellent reducibility. However, the passivation layer on ZVI surface limits the reactivity of ZVI. In this work, hydroxylamine hydrochloride (HA) has been selected to assist to improve the remediation efficiency of ZVI, and the removal rate constant of p-nitrophenol (PNP) increased nearly 90 times than that of ZVI alone. The effect of HA concentration on the degradation efficiency of PNP by ZVI was also discussed. The concentrations of Fe(Ⅱ) and Fe(Ⅲ) in reaction solution and on ZVI surface during the processes were measured. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) of fresh and used ZVI were characterized, which elucidated that HA could facilitate the reduction of Fe(Ⅲ) to Fe(Ⅱ), retard passivation, and extend the pH range of reaction. The mechanism of high degradation efficient in ZVI/HA system was studied through a series of probing experiments. Fe0 directly donated electron to reduce PNP, which was the fundamental reason. The contribution of Fe(Ⅱ) and active hydrogen generated on ZVI surface to the degradation efficiency of PNP could be negligible. It can be concluded that the addition of HA was an effective strategy to improve the reduction efficiency of ZVI.