Characteristics of zero-valent iron surface oxide films under the catalytic interface reactions by assisting ligands in nitrate-contaminated groundwater

Abstract

The surface passivation layer coating on zero-valent iron (ZVI) particles impedes the electron transfer from ZVI to nitrate. To enhance the efficiency of nitrate reduction by Fe(0), we tested the chemical process and the thickness of the iron oxide film on the surface of Fe(0) particles, utilizing Fe²⁺_aq in aqueous solution and wheat straw as ligands. A novel principal surface catalyzing reaction was formulated as follows: [Formula: see text] . When Fe²⁺_aq concentration increased from 0 - 200 mg·L^-1, the NO₃^- removal rate increased from 6.95% to 82.6% respectively during 12 h and it was 48%, 72%, 79% and 94% respectively in Fe⁰/WS ratio of 0, 0.25, 0.5 and 1 system. Uniform surface iron oxide films formed around the Fe(0) particles within 12 h after the adding Fe²⁺_aq or wheat straw to the Fe(0) system. The composition and thickness of these films were dependent on the quantity of added materials. X-ray diffraction (XRD) analysis revealed that surface oxide iron mainly consisted of Fe²⁺ or Fe³⁺ oxides, with Fe₃O₄ being predominant. The X-ray photoelectron spectroscopy (XPS) etching indicated that the addition of Fe(0)/straw at mass ratios of 1 or system with 20 mg·L^-1 Fe²⁺_aq resulted in the thinnest surface iron oxide layer. The study demonstrated that reducing the oxide layer's thickness was achieved through partial catalysis and enhanced complexation capacity. This reduction was facilitated by the introduction of Fe²⁺_aq or wheat straw into the Fe(0) system, potentially improving proton dissociation and promoting the ligand-assisted dissolution of Fe³⁺ oxides.

Keywords: Fe(2+) iron or Fe(3+) iron oxides; Iron corrosion; Ligand-assisted dissolution of Fe(3+) oxides; Surface iron oxide films.