Revisiting the iron oxide layer in micron zero-valent iron for pollutant removal: High contribution of surface-adsorbed Fe(II) and initiated protons

Abstract

Surface modification is an effective strategy for mitigating surface passivation and enhancing the reactivity of zero-valent iron(ZVI). The increased reactivity of modified ZVI is considered to be primarily attributed to enhanced electron transfer capability and generation of highly reactive electron donors on the modified ZVI surface, whereas the role of protons is often overlooked. In this research, a mild and rapid method for preparing Fe₃O₄-loaded micron ZVI(mZVI) composite material was proposed and the proton-related mechanism of the iron oxide layer was further investigated. Trace sulfide ions(S²⁻) were introduced to modulate the redox intensity, enabling rapid preparation of Fe₃O₄-loaded mZVI composite material(r-Fe₃O₄/mZVI) via aerial oxidation of Fe²⁺. XRD, XPS, and FTIR analyses confirmed that the iron oxide layer loaded on r-Fe₃O₄/mZVI was primarily composed of magnetite. 500 mg/L p-nitrophenol (PNP) was completely removed within 25 min by using 5 g/L r-Fe₃O₄/mZVI at an initial pH of 3. Cyclic voltammetry, alcohol quenching, products identification, and ferrozine inhibition experiments confirmed that surface-adsorbed Fe(II) was the reactive species. Quantitative calculation based on electron and proton mass balance shows that in r-Fe₃O₄/mZVI reaction, the oxidation of surface-adsorbed Fe(II) on the iron oxide layer provided 94.17 % of the protons and up to 31.39 % of the electrons required for PNP removal. Mechanistic analysis revealed that the iron oxide layer suppressed electron transfer from mZVI but promoted the oxidation of surface-adsorbed Fe(II), thereby enhancing proton generation. The pre-loading an iron oxide layer on mZVI surface to enhance proton generation represented an effective strategy for promoting pollutant removal from mZVI reduction.