Activation of ClO2 by Nanoscale Zero-Valent Iron for Efficient Soil Polycyclic Aromatic Hydrocarbon Degradation: New Insight into the Relative Contribution of Fe(IV) and Hydroxyl Radicals

Recently, the activation of chlorine dioxide (ClO2) by metal(oxide) for soil remediation has gained notable attention. However, the related activation mechanisms are still not clear. Herein, the variation of iron species and ClO2, the generated reactive oxygen species, and the toxicity of the degradation intermediates were explored and evaluated with nanoscale zero-valent iron (nFe0) being employed to activate ClO2 for soil polycyclic aromatic hydrocarbon (PAH) removal. With an optimized ClO2/nFe0 molar ratio of 15:1 and a soil/water ratio of 3:1, the degradation efficiency of phenanthrene improved 12% in comparison with that of a ClO2-alone system. The presence of nFe0 significantly promoted ClO2 consumption (improved 85.4%) but restrained ClO2- generation (reduced 22.5%). The surface Fe(II) and soluble Fe(II) in the ClO2/nFe0 system was 2.0-fold and 2.8-fold that in the nFe0 system after 2 min. Electron paramagnetic resonance analysis, along with quenching experiments, revealed that Fe(IV), HOCl, and •OH dominated phenanthrene degradation in a ClO2/nFe0 system, with oxidation contributions, respectively, of 34.3%, 52.8% and 12.9%. The degradation intermediates of PAHs in the ClO2/nFe0 system had lower estimated toxicity than those of the ClO2 system. The lettuces grown in ClO2/nFe0-treated soil displayed better results in bioassay indexes than those grown in ClO2-treated soil. This study offers new perspectives for the remediation of organic-pollutant-contaminated soil by using metal-activated ClO2 technology.