Simultaneous Cr(III)-EDTA decomplexation and Cr(III) sequestration by catalytic ozonation with sulfidated zero-valent iron: Kinetics and removal mechanism

Efficient removal of recalcitrant Cr(III)-organic complexes is always challenged by slow decomplexation process and the possible accumulation of highly toxic Cr(VI). Herein, a sulfidated zero-valent iron coupled ozonation strategy (S-ZVI/O₃) was proposed to achieve efficient decomplexation and simultaneous abatement of Cr(III)-EDTA without Cr(VI) accumulation. Results revealed that S-ZVI could catalyze O₃ to enhance removal of Cr(III)-EDTA and total organic carbon. Moreover, 88.3 % of total Cr was sequestrated by S-ZVI/O₃ and the corresponding kinetic constant was 3.1 times higher than that of ZVI/O₃. Mechanistically, electron paramagnetic resonance and probing tests verified HO^• was the dominant reactive oxidation species for decomplexation of Cr(III)-EDTA in both S-ZVI/O₃ and ZVI/O₃. More attractively, it was found that structural Fe(II) was the major O₃ activator in S-ZVI/O₃, whereas dissolved Fe²⁺ accounted for O₃ catalyzation in ZVI/O₃. The X-ray absorption spectroscopy analysis revealed that sulfidation treatment could enhance corrosion of ZVI with the formation of sufficient Fe(II) species. The in-situ formed Fe(II) could not only transform undesired Cr(VI) back to Cr(III) but also co-precipitate with Cr(III) to form solid Fe-Cr hydroxides. Besides Cr(III)-EDTA, S-ZVI/O₃ is also applicable to other EDTA complexed heavy metals. This work would provide a new method for the heavy metal complexes removal from water.