Ferrous ion mitigates the negative effects of humic acid on removal of 4-nitrophenol by zerovalent iron

In this study, Fe²⁺ addition was employed to overcome the negative effects of humic acid (HA) on contaminant removal by zerovalent iron (ZVI), and its feasibility to improve electron efficiency of ZVI was also tested. HA at high concentrations suppressed the removal of 4-nitrophenol (4-NP) by ZVI, while the addition of 0.25–1.0 mM Fe²⁺ could greatly mitigate this inhibitory effect and enhance 4-NP reduction. Specifically, with a mixed-order model, global fitting results showed that the addition of Fe²⁺ increased the rate constant from 0.124 × 10⁻²–0.219 × 10⁻² mM/min to 0.227 × 10⁻²–0.417 × 10⁻² mM/min and shortened lag period from 19.7–47.9 min to 8.0–15.2 min for 4-NP removal. The mechanistic investigation revealed this trend could be explained by the following aspects: i) Fe²⁺ can facilitate the generation of Fe(II)-containing oxides, which can act as an electron mediator or direct electron donor for 4-NP reduction; ii) the presence of Fe²⁺ could lead to aggregation of HA particles and accordingly reduced its coverage on ZVI surface. But the results of respike experiments indicate that Fe²⁺ addition did not show remarkable effect on the electron efficiency of 4-NP by ZVI, which should be associated with that Fe²⁺ was not able to favor the enrichment of 4-NP on ZVI surface.