Alkali synergistic sulfide-modified nZVI activation of persulfate for phenanthrene removal

Abstract

Sulfide-modified nano zero-valent iron (S-nZVI) and persulfate (PS) have been widely used in advanced oxidation processes, whereas their performance in alkaline solutions is rarely discussed. In this study, the synergistic activation mechanism of S-nZVI with alkali is proposed and verified by removal experiments and activator characterization. nZVI and S-nZVI with different sulfidation degrees were prepared and used to enhance PS oxidation for phenanthrene (PHE) removal. Results indicate that the S/Fe ratio significantly affects PHE degradation, with the optimal ratio at 0.28. S-nZVI exhibits superior degradation rates to nZVI over a wide pH range (3−11), especially in alkaline solutions with a maximum removal rate of 94.7% at pH = 9. At the same time, the relative synergistic enhancement ratio shows a maximum value of 1.80. Hydroxyl radicals are identified as the main active groups. X-ray photoelectron spectroscopy results suggest that the alkaline solutions promote the conversion of S2-, which increases from 24.7% to 33.1% compared to neutral solutions. Furthermore, the electrochemical tests of S-nZVI and nZVI reveal that S-nZVI has lower electron transfer impedance and higher oxidation peak current density at pH = 9, which probably elucidates that the alkali accelerates the electron conductivity of S-nZVI. These findings, for the first time, demonstrate the alkali synergistic activation mechanism of S-nZVI/PS systems and provide a deeper insight into the efficient degradation of persistent organic pollutants in complex pH environments.