Hydroxyl radical induced Cr flocculation via redox reaction: The extending application of heterogeneous advanced oxidation processes on Cr removal

Flocculation is a traditional and effective method to remove Cr from wastewater, but the addition of flocculants inevitably leads to secondary pollution. In this study, Cr flocculation was induced using hydroxyl radical (center dot OH) (center dot OH flocculation) generated in an electro-Fenton-like system, achieving total Cr removal of 98.68% at initial pH = 8 within 40 min. The obtained Cr flocs showed significantly higher Cr content, lower sludge yield, and good settling properties compared to alkali precipitation and polyaluminum chloride flocculation. center dot OH flocculation behaved like a typical flocculant, introducing electrostatic neutralization and bridging. The mechanism proposed that center dot OH could overcome the steric hindrance of Cr(H2O)63+ and combine with it as an additional ligand. Then Cr (III) was proved to undergo multi-step oxidation to form Cr(IV) and Cr(V). After these oxidation reactions, center dot OH flocculation took precedence over Cr(VI) generation. As a result, Cr(VI) didn't accumulate in solution until center dot OH flocculation was completed. This work provided a clean and eco-friendly strategy for Cr flocculation instead of flocculants and extended the application of advanced oxidation processes (AOPs), which is expected to enrich existing strategies of AOPs towards Cr removal.