Interactions of iron-based nanoparticles with soil dissolved organic matter: adsorption, aging, and effects on hexavalent chromium removal

The interactions and mechanisms between soil dissolved organic matter (DOM) and three types of iron-based nanoparticles (NPs), i.e., nanoscale zero-valent iron (nZVI) particles, Fe₂O₃ NPs, and Fe₃O₄ NPs, were investigated in short-term exposure experiments. The adsorption results showed that soil DOM was rapidly adsorbed on the surface of the iron-based NPs with the adsorption rate varying according to Fe₃O₄ > Fe₂O₃ > nZVI. Spectral analysis results revealed that aromatic DOM fractions with high-molecular-weights were preferentially adsorbed. The binding mechanism was determined as hydrogen bonding and ligand exchange via Fourier transform infrared spectroscopy (FT-IR) analysis. Scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy, and X-ray diffraction were used to identify the corrosion products of the three iron-based NPs at the adsorption equilibrium. The results suggest that Fe₃O₄ and/or γ-Fe₂O₃ and α-FeOOH were the main corrosion products of nZVIs and α-FeOOH was obtained as an aged product of Fe₃O₄ NPs. Results of Cr(VI) removal tests suggest that the aged nZVI achieved 79.87% of Cr(VI) removal and the Cr(VI) removal efficiency was significantly improved by coating DOM onto Fe₂O₃ NPs. The overall data indicate the fate and transformation of iron-based NPs and the enhancement for Cr(VI) removal after interactions between DOM and NPs.