The Ce(IV) boosted superior coagulation: A performance evaluation and mechanism analysis in Ce-based salts

Research on inorganic metal coagulants has largely focused on the Al, Fe, Ti and Zr-based salts, leaving insufficient studies on the metal Ce. This study first attempted to conduct an in-depth investigation into the coagulation effect of Ce-based reagents with different valence states. The CCl was a pure Ce(III)-based salt, while Ce(IV)/Ce(III) in CAN equaled 2.7/1. The results demonstrated that CAN possessed far superior capability of turbidity removal compared with CCl, which was on par with polyaluminum chloride (PAC). And its ability   to   remove   organic matter even surpassed that of PAC. Furthermore, CAN exhibited more remarkable and stable performance across a broad pH range. Mechanism studies confirmed that the Ce(III) had the strongest organic ligand complexing ability but insufficient hydrolysis ability, leading the complex re-stabilized in solution. Whereas, the merits of Ce(IV) was based on the following distinctive properties: (1) the high positive charge, with zeta potential even close to that of PAC under acidic condition, (2) the readily formed hydrolysis products owning film-like structure to enhance sweep flocculation, (3) the high coordination number to offer abundant reaction sites to balance the three effects, electric neutralization, complexation, and sweeping, to fully exploit their respective advantages. Thus, the above three effects were combined to achieve the stable coagulation efficiency in CAN containing both Ce(IV) and Ce(III), which might be insufficient or competitive in PAC or CCl. The clarification of the coagulation mechanisms is conducive to better developing Ce-based coagulants and may offer valuable insights into finding more suitable metal-based reagents.